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Clean Code PHP 8.4

Table of Contents

Introduction
Variables
Comparison
Functions
Objects and Data Structures
Classes
SOLID
Don't repeat yourself (DRY)
Modern PHP Features

Introduction

Software engineering principles, from Robert C. Martin's book Clean Code, adapted for PHP 8.4. This is not a style guide. It's a guide to producing readable, reusable, and refactorable software in PHP.

Not every principle herein has to be strictly followed, and even fewer will be universally agreed upon. These are guidelines and nothing more, but they are ones codified over many years of collective experience by the authors of Clean Code.

Inspired from clean-code-javascript.

This guide focuses on PHP 8.4+ features and modern syntax to write cleaner, more maintainable code.

Variables

Use meaningful and pronounceable variable names

Bad:

$ymdstr = $moment->format('y-m-d');

Good:

$currentDate = $moment->format('y-m-d');

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Use the same vocabulary for the same type of variable

Bad:

function getUserInfo(): array { /* ... */ }
function getUserData(): array { /* ... */ }
function getUserRecord(): array { /* ... */ }
function getUserProfile(): array { /* ... */ }

Good:

function getUser(): User
{
    // Return a proper User object instead of array
}

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Use searchable names (part 1)

We will read more code than we will ever write. It's important that the code we do write is readable and searchable. By not naming variables that end up being meaningful for understanding our program, we hurt our readers. Make your names searchable.

Bad:

// What the heck is 448 for?
$result = $serializer->serialize($data, 448);

Good:

$result = $serializer->serialize(
    $data, 
    JSON_UNESCAPED_SLASHES | JSON_PRETTY_PRINT | JSON_UNESCAPED_UNICODE
);

Use searchable names (part 2)

Bad:

class User
{
    // What the heck is 7 for?
    public int $access = 7;
}

// What the heck is 4 for?
if ($user->access & 4) {
    // ...
}

// What's going on here?
$user->access ^= 2;

Good:

enum UserPermission: int
{
    case READ = 1;
    case CREATE = 2;
    case UPDATE = 4;
    case DELETE = 8;
}

class User
{
    // User as default can read, create and update something
    public int $access = UserPermission::READ->value | 
                        UserPermission::CREATE->value | 
                        UserPermission::UPDATE->value;
}

if ($user->access & UserPermission::UPDATE->value) {
    // do edit ...
}

// Deny access rights to create something
$user->access ^= UserPermission::CREATE->value;

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Use explanatory variables

Bad:

$address = 'One Infinite Loop, Cupertino 95014';
$cityZipCodeRegex = '/^[^,]+,\s*(.+?)\s*(\d{5})$/';
preg_match($cityZipCodeRegex, $address, $matches);

saveCityZipCode($matches[1], $matches[2]);

Not bad:

It's better, but we are still heavily dependent on regex.

$address = 'One Infinite Loop, Cupertino 95014';
$cityZipCodeRegex = '/^[^,]+,\s*(.+?)\s*(\d{5})$/';
preg_match($cityZipCodeRegex, $address, $matches);

[, $city, $zipCode] = $matches;
saveCityZipCode($city, $zipCode);

Good:

Decrease dependence on regex by naming subpatterns.

$address = 'One Infinite Loop, Cupertino 95014';
$cityZipCodeRegex = '/^[^,]+,\s*(?<city>.+?)\s*(?<zipCode>\d{5})$/';
preg_match($cityZipCodeRegex, $address, $matches);

saveCityZipCode($matches['city'], $matches['zipCode']);

Best (PHP 8.4):

Use structured approach with value objects.

readonly class Address
{
    public function __construct(
        public string $street,
        public string $city,
        public string $zipCode
    ) {}
    
    public static function fromString(string $addressString): self
    {
        $regex = '/^(?<street>[^,]+),\s*(?<city>.+?)\s*(?<zipCode>\d{5})$/';
        preg_match($regex, $addressString, $matches);
        
        return new self($matches['street'], $matches['city'], $matches['zipCode']);
    }
}

$address = Address::fromString('One Infinite Loop, Cupertino 95014');
saveCityZipCode($address->city, $address->zipCode);

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Avoid nesting too deeply and return early (part 1)

Too many if-else statements can make your code hard to follow. Explicit is better than implicit.

Bad:

function isShopOpen($day): bool
{
    if ($day) {
        if (is_string($day)) {
            $day = strtolower($day);
            if ($day === 'friday') {
                return true;
            } elseif ($day === 'saturday') {
                return true;
            } elseif ($day === 'sunday') {
                return true;
            }
            return false;
        }
        return false;
    }
    return false;
}

Good:

function isShopOpen(?string $day): bool
{
    if (empty($day)) {
        return false;
    }

    $openingDays = ['friday', 'saturday', 'sunday'];

    return in_array(strtolower($day), $openingDays, true);
}

Best (PHP 8.4):

enum WeekDay: string
{
    case MONDAY = 'monday';
    case TUESDAY = 'tuesday';
    case WEDNESDAY = 'wednesday';
    case THURSDAY = 'thursday';
    case FRIDAY = 'friday';
    case SATURDAY = 'saturday';
    case SUNDAY = 'sunday';
}

function isShopOpen(?WeekDay $day): bool
{
    return match($day) {
        WeekDay::FRIDAY, WeekDay::SATURDAY, WeekDay::SUNDAY => true,
        default => false,
    };
}

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Avoid nesting too deeply and return early (part 2)

Bad:

function fibonacci(int $n)
{
    if ($n < 50) {
        if ($n !== 0) {
            if ($n !== 1) {
                return fibonacci($n - 1) + fibonacci($n - 2);
            }
            return 1;
        }
        return 0;
    }
    return 'Not supported';
}

Good:

function fibonacci(int $n): int
{
    return match(true) {
        $n === 0 || $n === 1 => $n,
        $n >= 50 => throw new InvalidArgumentException('Not supported'),
        default => fibonacci($n - 1) + fibonacci($n - 2),
    };
}

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Avoid Mental Mapping

Don't force the reader of your code to translate what the variable means. Explicit is better than implicit.

Bad:

$l = ['Austin', 'New York', 'San Francisco'];

for ($i = 0; $i < count($l); $i++) {
    $li = $l[$i];
    doStuff();
    doSomeOtherStuff();
    // ...
    // ...
    // ...
    // Wait, what is `$li` for again?
    dispatch($li);
}

Good:

$locations = ['Austin', 'New York', 'San Francisco'];

foreach ($locations as $location) {
    doStuff();
    doSomeOtherStuff();
    // ...
    // ...
    // ...
    dispatch($location);
}

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Don't add unneeded context

If your class/object name tells you something, don't repeat that in your variable name.

Bad:

class Car
{
    public string $carMake;
    public string $carModel;
    public string $carColour;
}

Good:

readonly class Car
{
    public function __construct(
        public string $make,
        public string $model,
        public string $colour
    ) {}
}

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Comparison

Use identical comparison

Not good:

The simple comparison will convert the string into an integer.

$a = '42';
$b = 42;

if ($a != $b) {
    // The expression will always pass
}

The comparison $a != $b returns FALSE but in fact it's TRUE! The string 42 is different than the integer 42.

Good:

The identical comparison will compare type and value.

$a = '42';
$b = 42;

if ($a !== $b) {
    // The expression is verified
}

The comparison $a !== $b returns TRUE.

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Null coalescing operator

Null coalescing is an operator introduced in PHP 7. The null coalescing operator ?? returns its first operand if it exists and is not null; otherwise it returns its second operand.

Bad:

if (isset($_GET['name'])) {
    $name = $_GET['name'];
} elseif (isset($_POST['name'])) {
    $name = $_POST['name'];
} else {
    $name = 'nobody';
}

Good:

$name = $_GET['name'] ?? $_POST['name'] ?? 'nobody';

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Null coalescing assignment operator

PHP 7.4 introduced the null coalescing assignment operator ??=.

Bad:

$config['host'] = $config['host'] ?? 'localhost';
$config['port'] = $config['port'] ?? 3306;

Good:

$config['host'] ??= 'localhost';
$config['port'] ??= 3306;

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Match expressions

The match expression (PHP 8+) is more powerful than switch statements.

Bad:

function getHttpStatusMessage(int $code): string
{
    switch ($code) {
        case 200:
            return 'OK';
        case 404:
            return 'Not Found';
        case 500:
            return 'Internal Server Error';
        default:
            return 'Unknown';
    }
}

Good:

function getHttpStatusMessage(int $code): string
{
    return match($code) {
        200 => 'OK',
        404 => 'Not Found',
        500 => 'Internal Server Error',
        default => 'Unknown',
    };
}

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Functions

Use default arguments instead of short circuiting or conditionals

Not good:

This is not good because $breweryName can be NULL.

function createMicrobrewery($breweryName = 'Hipster Brew Co.'): void
{
    // ...
}

Good:

You can use type hinting and be sure that the $breweryName will not be NULL.

function createMicrobrewery(string $breweryName = 'Hipster Brew Co.'): void
{
    // ...
}

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Function arguments (2 or fewer ideally)

Limiting the amount of function parameters is incredibly important because it makes testing your function easier. Having more than three leads to a combinatorial explosion where you have to test tons of different cases with each separate argument.

Zero arguments is the ideal case. One or two arguments is ok, and three should be avoided. Anything more than that should be consolidated. Usually, if you have more than two arguments then your function is trying to do too much. In cases where it's not, most of the time a higher-level object will suffice as an argument.

Bad:

class Questionnaire
{
    public function __construct(
        string $firstname,
        string $lastname,
        string $patronymic,
        string $region,
        string $district,
        string $city,
        string $phone,
        string $email
    ) {
        // ...
    }
}

Good:

readonly class Name
{
    public function __construct(
        public string $firstname,
        public string $lastname,
        public string $patronymic
    ) {}
}

readonly class City
{
    public function __construct(
        public string $region,
        public string $district,
        public string $city
    ) {}
}

readonly class Contact
{
    public function __construct(
        public string $phone,
        public string $email
    ) {}
}

readonly class Questionnaire
{
    public function __construct(
        public Name $name,
        public City $city,
        public Contact $contact
    ) {}
}

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Function names should say what they do

Bad:

class Email
{
    public function handle(): void
    {
        mail($this->to, $this->subject, $this->body);
    }
}

$message = new Email(/* ... */);
// What is this? A handle for the message? Are we writing to a file now?
$message->handle();

Good:

class Email
{
    public function send(): void
    {
        mail($this->to, $this->subject, $this->body);
    }
}

$message = new Email(/* ... */);
// Clear and obvious
$message->send();

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Functions should only be one level of abstraction

When you have more than one level of abstraction your function is usually doing too much. Splitting up functions leads to reusability and easier testing.

Bad:

function parseBetterPHPAlternative(string $code): void
{
    $regexes = [
        // ...
    ];

    $statements = explode(' ', $code);
    $tokens = [];
    foreach ($regexes as $regex) {
        foreach ($statements as $statement) {
            // ...
        }
    }

    $ast = [];
    foreach ($tokens as $token) {
        // lex...
    }

    foreach ($ast as $node) {
        // parse...
    }
}

Good:

readonly class Tokenizer
{
    public function tokenize(string $code): array
    {
        $regexes = [
            // ...
        ];

        $statements = explode(' ', $code);
        $tokens = [];
        foreach ($regexes as $regex) {
            foreach ($statements as $statement) {
                $tokens[] = /* ... */;
            }
        }

        return $tokens;
    }
}

readonly class Lexer
{
    public function lexify(array $tokens): array
    {
        $ast = [];
        foreach ($tokens as $token) {
            $ast[] = /* ... */;
        }

        return $ast;
    }
}

readonly class BetterPHPAlternative
{
    public function __construct(
        private Tokenizer $tokenizer,
        private Lexer $lexer
    ) {}

    public function parse(string $code): void
    {
        $tokens = $this->tokenizer->tokenize($code);
        $ast = $this->lexer->lexify($tokens);
        foreach ($ast as $node) {
            // parse...
        }
    }
}

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Don't use flags as function parameters

Flags tell your user that this function does more than one thing. Functions should do one thing. Split out your functions if they are following different code paths based on a boolean.

Bad:

function createFile(string $name, bool $temp = false): void
{
    if ($temp) {
        touch('./temp/' . $name);
    } else {
        touch($name);
    }
}

Good:

function createFile(string $name): void
{
    touch($name);
}

function createTempFile(string $name): void
{
    touch('./temp/' . $name);
}

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Avoid Side Effects

A function produces a side effect if it does anything other than take a value in and return another value or values. A side effect could be writing to a file, modifying some global variable, or accidentally wiring all your money to a stranger.

Now, you do need to have side effects in a program on occasion. Like the previous example, you might need to write to a file. What you want to do is to centralise where you are doing this. Don't have several functions and classes that write to a particular file. Have one service that does it. One and only one.

The main point is to avoid common pitfalls like sharing state between objects without any structure, using mutable data types that can be written to by anything, and not centralising where your side effects occur. If you can do this, you will be happier than the vast majority of other programmers.

Bad:

// Global variable referenced by following function.
// If we had another function that used this name, now it'd be an array and it could break it.
$name = 'Ryan McDermott';

function splitIntoFirstAndLastName(): void
{
    global $name;

    $name = explode(' ', $name);
}

splitIntoFirstAndLastName();

var_dump($name);
// ['Ryan', 'McDermott'];

Good:

function splitIntoFirstAndLastName(string $name): array
{
    return explode(' ', $name);
}

$name = 'Ryan McDermott';
$newName = splitIntoFirstAndLastName($name);

var_dump($name);
// 'Ryan McDermott';

var_dump($newName);
// ['Ryan', 'McDermott'];

Best (PHP 8.4):

readonly class PersonName
{
    public function __construct(
        public string $firstName,
        public string $lastName
    ) {}
    
    public static function fromFullName(string $fullName): self
    {
        [$firstName, $lastName] = explode(' ', $fullName, 2);
        return new self($firstName, $lastName);
    }
    
    public function getFullName(): string
    {
        return "{$this->firstName} {$this->lastName}";
    }
}

$originalName = 'Ryan McDermott';
$personName = PersonName::fromFullName($originalName);

var_dump($originalName); // 'Ryan McDermott'
var_dump($personName->firstName); // 'Ryan'
var_dump($personName->lastName); // 'McDermott'

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Don't write to global functions

Polluting globals is a bad practice in many languages because you could clash with another library and the user of your API would be none-the-wiser until they get an exception in production. Let's think about an example: what if you wanted to have configuration array? You could write global function like config(), but it could clash with another library that tried to do the same thing.

Bad:

function config(): array
{
    return [
        'foo' => 'bar',
    ];
}

Good:

readonly class Configuration
{
    public function __construct(
        private array $configuration
    ) {}

    public function get(string $key): mixed
    {
        return $this->configuration[$key] ?? null;
    }
}

$configuration = new Configuration([
    'foo' => 'bar',
]);

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Don't use a Singleton pattern

Singleton is an anti-pattern. It violates several SOLID principles and makes testing difficult.

Bad:

class DBConnection
{
    private static ?self $instance = null;

    private function __construct(private string $dsn)
    {
        // ...
    }

    public static function getInstance(): self
    {
        if (self::$instance === null) {
            self::$instance = new self('default_dsn');
        }

        return self::$instance;
    }
}

$singleton = DBConnection::getInstance();

Good:

readonly class DBConnection
{
    public function __construct(
        private string $dsn
    ) {}
    
    // ... connection methods
}

// Use dependency injection
$connection = new DBConnection($dsn);

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Encapsulate conditionals

Bad:

if ($article->state === 'published') {
    // ...
}

Good:

if ($article->isPublished()) {
    // ...
}

Best (PHP 8.4):

enum ArticleState: string
{
    case DRAFT = 'draft';
    case PUBLISHED = 'published';
    case ARCHIVED = 'archived';
}

readonly class Article
{
    public function __construct(
        public string $title,
        public string $content,
        public ArticleState $state = ArticleState::DRAFT
    ) {}
    
    public function isPublished(): bool
    {
        return $this->state === ArticleState::PUBLISHED;
    }
}

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Avoid negative conditionals

Bad:

function isDOMNodeNotPresent(\DOMNode $node): bool
{
    // ...
}

if (!isDOMNodeNotPresent($node)) {
    // ...
}

Good:

function isDOMNodePresent(\DOMNode $node): bool
{
    // ...
}

if (isDOMNodePresent($node)) {
    // ...
}

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Avoid conditionals

This seems like an impossible task. Upon first hearing this, most people say, "how am I supposed to do anything without an if statement?" The answer is that you can use polymorphism to achieve the same task in many cases. The second question is usually, "well that's great but why would I want to do that?" The answer is a previous clean code concept we learned: a function should only do one thing. When you have classes and functions that have if statements, you are telling your user that your function does more than one thing. Remember, just do one thing.

Bad:

class Airplane
{
    public function getCruisingAltitude(): int
    {
        return match ($this->type) {
            '777' => $this->getMaxAltitude() - $this->getPassengerCount(),
            'Air Force One' => $this->getMaxAltitude(),
            'Cessna' => $this->getMaxAltitude() - $this->getFuelExpenditure(),
            default => throw new InvalidArgumentException('Unknown aircraft type'),
        };
    }
}

Good:

interface Airplane
{
    public function getCruisingAltitude(): int;
}

readonly class Boeing777 implements Airplane
{
    public function __construct(
        private int $maxAltitude,
        private int $passengerCount
    ) {}

    public function getCruisingAltitude(): int
    {
        return $this->maxAltitude - $this->passengerCount;
    }
}

readonly class AirForceOne implements Airplane
{
    public function __construct(
        private int $maxAltitude
    ) {}

    public function getCruisingAltitude(): int
    {
        return $this->maxAltitude;
    }
}

readonly class Cessna implements Airplane
{
    public function __construct(
        private int $maxAltitude,
        private int $fuelExpenditure
    ) {}

    public function getCruisingAltitude(): int
    {
        return $this->maxAltitude - $this->fuelExpenditure;
    }
}

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Avoid type-checking (part 1)

PHP has strong typing capabilities in modern versions. Use them instead of manual type checking.

Bad:

function travelToTexas($vehicle): void
{
    if ($vehicle instanceof Bicycle) {
        $vehicle->pedalTo(new Location('texas'));
    } elseif ($vehicle instanceof Car) {
        $vehicle->driveTo(new Location('texas'));
    }
}

Good:

interface Vehicle
{
    public function travelTo(Location $location): void;
}

function travelToTexas(Vehicle $vehicle): void
{
    $vehicle->travelTo(new Location('texas'));
}

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Avoid type-checking (part 2)

If you are working with basic primitive values like strings, integers, and arrays, use PHP's type declarations and strict mode.

Bad:

function combine($val1, $val2): int
{
    if (!is_numeric($val1) || !is_numeric($val2)) {
        throw new InvalidArgumentException('Must be of type Number');
    }

    return $val1 + $val2;
}

Good:

function combine(int|float $val1, int|float $val2): int|float
{
    return $val1 + $val2;
}

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Remove dead code

Dead code is just as bad as duplicate code. There's no reason to keep it in your codebase. If it's not being called, get rid of it! It will still be safe in your version history if you still need it.

Bad:

function oldRequestModule(string $url): void
{
    // ...
}

function newRequestModule(string $url): void
{
    // ...
}

$request = newRequestModule($requestUrl);
inventoryTracker('apples', $request, 'www.inventory-awesome.io');

Good:

function requestModule(string $url): void
{
    // ...
}

$request = requestModule($requestUrl);
inventoryTracker('apples', $request, 'www.inventory-awesome.io');

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Objects and Data Structures

Use object encapsulation

In PHP you can set public, protected and private keywords for methods. Using it, you can control properties modification on an object.

When you want to do more beyond getting an object property, you don't have to look up and change every accessor in your codebase.
Makes adding validation simple when doing a set.
Encapsulates the internal representation.
Easy to add logging and error handling when getting and setting.
Inheriting this class, you can override default functionality.
You can lazy load your object's properties, let's say getting it from a server.

Additionally, this is part of Open/Closed principle.

Bad:

class BankAccount
{
    public int $balance = 1000;
}

$bankAccount = new BankAccount();

// Buy shoes...
$bankAccount->balance -= 100;

Good:

class BankAccount
{
    public function __construct(
        private int $balance = 1000
    ) {}

    public function withdraw(int $amount): void
    {
        if ($amount > $this->balance) {
            throw new InvalidArgumentException('Amount greater than available balance.');
        }

        $this->balance -= $amount;
    }

    public function deposit(int $amount): void
    {
        $this->balance += $amount;
    }

    public function getBalance(): int
    {
        return $this->balance;
    }
}

$bankAccount = new BankAccount();

// Buy shoes...
$bankAccount->withdraw($shoesPrice);

// Get balance
$balance = $bankAccount->getBalance();

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Make objects have private/protected members

public methods and properties are most dangerous for changes, because some outside code may easily rely on them and you can't control what code relies on them. Modifications in class are dangerous for all users of class.
protected modifier are as dangerous as public, because they are available in scope of any child class. This effectively means that difference between public and protected is only in access mechanism, but encapsulation guarantee remains the same. Modifications in class are dangerous for all descendant classes.
private modifier guarantees that code is dangerous to modify only in boundaries of single class (you are safe for modifications and you won't have Jenga effect).

Therefore, use private by default and public/protected when you need to provide access for external classes.

Bad:

class Employee
{
    public string $name;

    public function __construct(string $name)
    {
        $this->name = $name;
    }
}

$employee = new Employee('John Doe');
// Employee name: John Doe
echo 'Employee name: ' . $employee->name;

Good:

readonly class Employee
{
    public function __construct(
        private string $name
    ) {}

    public function getName(): string
    {
        return $this->name;
    }
}

$employee = new Employee('John Doe');
// Employee name: John Doe
echo 'Employee name: ' . $employee->getName();

⬆ back to top

Use readonly properties

PHP 8.1 introduced readonly properties, and PHP 8.2 introduced readonly classes. Use them to create immutable objects.

Bad:

class Point
{
    private float $x;
    private float $y;

    public function __construct(float $x, float $y)
    {
        $this->x = $x;
        $this->y = $y;
    }

    public function getX(): float
    {
        return $this->x;
    }

    public function getY(): float
    {
        return $this->y;
    }
}

Good:

readonly class Point
{
    public function __construct(
        public float $x,
        public float $y
    ) {}
    
    public function distanceFrom(Point $other): float
    {
        return sqrt(($this->x - $other->x) ** 2 + ($this->y - $other->y) ** 2);
    }
}

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Classes

Prefer composition over inheritance

As stated famously in Design Patterns by the Gang of Four, you should prefer composition over inheritance where you can. There are lots of good reasons to use inheritance and lots of good reasons to use composition. The main point for this maxim is that if your mind instinctively goes for inheritance, try to think if composition could model your problem better. In some cases it can.

You might be wondering then, "when should I use inheritance?" It depends on your problem at hand, but this is a decent list of when inheritance makes more sense than composition:

Your inheritance represents an "is-a" relationship and not a "has-a" relationship (Human->Animal vs. User->UserDetails).
You can reuse code from the base classes (Humans can move like all animals).
You want to make global changes to derived classes by changing a base class. (Change the caloric expenditure of all animals when they move).

Bad:

class Employee
{
    public function __construct(
        private string $name,
        private string $email
    ) {}
}

// Bad because Employees "have" tax data.
// EmployeeTaxData is not a type of Employee
class EmployeeTaxData extends Employee
{
    public function __construct(
        string $name,
        string $email,
        private string $ssn,
        private string $salary
    ) {
        parent::__construct($name, $email);
    }
}

Good:

readonly class EmployeeTaxData
{
    public function __construct(
        public string $ssn,
        public string $salary
    ) {}
}

readonly class Employee
{
    public function __construct(
        public string $name,
        public string $email,
        public ?EmployeeTaxData $taxData = null
    ) {}
    
    public function setTaxData(EmployeeTaxData $taxData): self
    {
        return new self($this->name, $this->email, $taxData);
    }
}

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Avoid fluent interfaces

A Fluent interface is an object oriented API that aims to improve the readability of the source code by using Method chaining.

While there can be some contexts, frequently builder objects, where this pattern reduces the verbosity of the code, more often it comes at some costs:

Breaks Encapsulation.
Breaks Decorators.
Is harder to mock in a test suite.
Makes diffs of commits harder to read.

Bad:

class Car
{
    private string $make = 'Honda';
    private string $model = 'Accord';
    private string $colour = 'white';

    public function setMake(string $make): self
    {
        $this->make = $make;
        return $this;
    }

    public function setModel(string $model): self
    {
        $this->model = $model;
        return $this;
    }

    public function setColour(string $colour): self
    {
        $this->colour = $colour;
        return $this;
    }

    public function dump(): void
    {
        var_dump($this->make, $this->model, $this->colour);
    }
}

$car = (new Car())
    ->setColour('pink')
    ->setMake('Ford')
    ->setModel('F-150')
    ->dump();

Good:

readonly class Car
{
    public function __construct(
        public string $make = 'Honda',
        public string $model = 'Accord',
        public string $colour = 'white'
    ) {}

    public function withMake(string $make): self
    {
        return new self($make, $this->model, $this->colour);
    }

    public function withModel(string $model): self
    {
        return new self($this->make, $model, $this->colour);
    }

    public function withColour(string $colour): self
    {
        return new self($this->make, $this->model, $colour);
    }

    public function dump(): void
    {
        var_dump($this->make, $this->model, $this->colour);
    }
}

$car = (new Car())
    ->withColour('pink')
    ->withMake('Ford')
    ->withModel('F-150');
$car->dump();

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Prefer final classes

The final keyword should be used whenever possible:

It prevents an uncontrolled inheritance chain.
It encourages composition.
It encourages the Single Responsibility Principle.
It encourages developers to use your public methods instead of extending the class to get access to protected ones.
It allows you to change your code without breaking applications that use your class.

The only condition is that your class should implement an interface and no other public methods are defined.

Bad:

final class Car
{
    public function __construct(
        private string $colour
    ) {}

    public function getColour(): string
    {
        return $this->colour;
    }
}

Good:

interface Vehicle
{
    public function getColour(): string;
}

final readonly class Car implements Vehicle
{
    public function __construct(
        private string $colour
    ) {}

    public function getColour(): string
    {
        return $this->colour;
    }
}

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Use property promotion

PHP 8.0 introduced constructor property promotion, which reduces boilerplate code significantly.

Bad:

class User
{
    private string $name;
    private string $email;
    private int $age;

    public function __construct(string $name, string $email, int $age)
    {
        $this->name = $name;
        $this->email = $email;
        $this->age = $age;
    }

    public function getName(): string
    {
        return $this->name;
    }

    public function getEmail(): string
    {
        return $this->email;
    }

    public function getAge(): int
    {
        return $this->age;
    }
}

Good:

readonly class User
{
    public function __construct(
        public string $name,
        public string $email,
        public int $age
    ) {}
}

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Use enums for constants

PHP 8.1 introduced enums, which are perfect for representing a fixed set of possible values.

Bad:

class OrderStatus
{
    public const PENDING = 'pending';
    public const CONFIRMED = 'confirmed';
    public const SHIPPED = 'shipped';
    public const DELIVERED = 'delivered';
    public const CANCELLED = 'cancelled';
}

class Order
{
    public function __construct(
        public string $status = OrderStatus::PENDING
    ) {}
}

Good:

enum OrderStatus: string
{
    case PENDING = 'pending';
    case CONFIRMED = 'confirmed';
    case SHIPPED = 'shipped';
    case DELIVERED = 'delivered';
    case CANCELLED = 'cancelled';
    
    public function canBeModified(): bool
    {
        return match($this) {
            self::PENDING, self::CONFIRMED => true,
            default => false,
        };
    }
}

readonly class Order
{
    public function __construct(
        public OrderStatus $status = OrderStatus::PENDING
    ) {}
    
    public function canModify(): bool
    {
        return $this->status->canBeModified();
    }
}

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SOLID

SOLID is the mnemonic acronym introduced by Michael Feathers for the first five principles named by Robert Martin, which meant five basic principles of object-oriented programming and design.

S: Single Responsibility Principle (SRP)
O: Open/Closed Principle (OCP)
L: Liskov Substitution Principle (LSP)
I: Interface Segregation Principle (ISP)
D: Dependency Inversion Principle (DIP)

Single Responsibility Principle (SRP)

As stated in Clean Code, "There should never be more than one reason for a class to change". It's tempting to jam-pack a class with a lot of functionality, like when you can only take one suitcase on your flight. The issue with this is that your class won't be conceptually cohesive and it will give it many reasons to change. Minimising the amount of times you need to change a class is important. It's important because if too much functionality is in one class and you modify a piece of it, it can be difficult to understand how that will affect other dependent modules in your codebase.

Bad:

class UserSettings
{
    public function __construct(
        private User $user
    ) {}

    public function changeSettings(array $settings): void
    {
        if ($this->verifyCredentials()) {
            // ...
        }
    }

    private function verifyCredentials(): bool
    {
        // ...
    }
}

Good:

readonly class UserAuth
{
    public function __construct(
        private User $user
    ) {}

    public function verifyCredentials(): bool
    {
        // ...
    }
}

readonly class UserSettings
{
    public function __construct(
        private User $user,
        private UserAuth $auth
    ) {}

    public function changeSettings(array $settings): void
    {
        if ($this->auth->verifyCredentials()) {
            // ...
        }
    }
}

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Open/Closed Principle (OCP)

As stated by Bertrand Meyer, "software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification." What does that mean though? This principle basically states that you should allow users to add new functionalities without changing existing code.

Bad:

abstract class Adapter
{
    abstract public function getName(): string;
}

class AjaxAdapter extends Adapter
{
    public function getName(): string
    {
        return 'ajaxAdapter';
    }
}

class NodeAdapter extends Adapter
{
    public function getName(): string
    {
        return 'nodeAdapter';
    }
}

class HttpRequester
{
    public function __construct(
        private Adapter $adapter
    ) {}

    public function fetch(string $url): Promise
    {
        return match ($this->adapter->getName()) {
            'ajaxAdapter' => $this->makeAjaxCall($url),
            'httpNodeAdapter' => $this->makeHttpCall($url),
            default => throw new InvalidArgumentException('Unknown adapter'),
        };
    }

    private function makeAjaxCall(string $url): Promise
    {
        // request and return promise
    }

    private function makeHttpCall(string $url): Promise
    {
        // request and return promise
    }
}

Good:

interface Adapter
{
    public function request(string $url): Promise;
}

final readonly class AjaxAdapter implements Adapter
{
    public function request(string $url): Promise
    {
        // request and return promise
    }
}

final readonly class NodeAdapter implements Adapter
{
    public function request(string $url): Promise
    {
        // request and return promise
    }
}

final readonly class HttpRequester
{
    public function __construct(
        private Adapter $adapter
    ) {}

    public function fetch(string $url): Promise
    {
        return $this->adapter->request($url);
    }
}

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Liskov Substitution Principle (LSP)

This is a scary term for a very simple concept. It's formally defined as "If S is a subtype of T, then objects of type T may be replaced with objects of type S (i.e., objects of type S may substitute objects of type T) without altering any of the desirable properties of that program (correctness, task performed, etc.)." That's an even scarier definition.

The best explanation for this is if you have a parent class and a child class, then the base class and child class can be used interchangeably without getting incorrect results. This might still be confusing, so let's take a look at the classic Square-Rectangle example. Mathematically, a square is a rectangle, but if you model it using the "is-a" relationship via inheritance, you quickly get into trouble.

Bad:

class Rectangle
{
    public function __construct(
        protected int $width = 0,
        protected int $height = 0
    ) {}

    public function setWidth(int $width): void
    {
        $this->width = $width;
    }

    public function setHeight(int $height): void
    {
        $this->height = $height;
    }

    public function getArea(): int
    {
        return $this->width * $this->height;
    }
}

class Square extends Rectangle
{
    public function setWidth(int $width): void
    {
        $this->width = $this->height = $width;
    }

    public function setHeight(int $height): void
    {
        $this->width = $this->height = $height;
    }
}

function printArea(Rectangle $rectangle): void
{
    $rectangle->setWidth(4);
    $rectangle->setHeight(5);

    // BAD: Will return 25 for Square. Should be 20.
    echo sprintf('%s has area %d.', get_class($rectangle), $rectangle->getArea()) . PHP_EOL;
}

$rectangles = [new Rectangle(), new Square()];

foreach ($rectangles as $rectangle) {
    printArea($rectangle);
}

Good:

The best way is separate the quadrangles and allocation of a more general subtype for both shapes.

interface Shape
{
    public function getArea(): int;
}

final readonly class Rectangle implements Shape
{
    public function __construct(
        public int $width,
        public int $height
    ) {}

    public function getArea(): int
    {
        return $this->width * $this->height;
    }
}

final readonly class Square implements Shape
{
    public function __construct(
        public int $length
    ) {}

    public function getArea(): int
    {
        return $this->length ** 2;
    }
}

function printArea(Shape $shape): void
{
    echo sprintf('%s has area %d.', get_class($shape), $shape->getArea()) . PHP_EOL;
}

$shapes = [new Rectangle(4, 5), new Square(5)];

foreach ($shapes as $shape) {
    printArea($shape);
}

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Interface Segregation Principle (ISP)

ISP states that "Clients should not be forced to depend upon interfaces that they do not use."

A good example to look at that demonstrates this principle is for classes that require large settings objects. Not requiring clients to set up huge amounts of options is beneficial, because most of the time they won't need all of the settings. Making them optional helps prevent having a "fat interface".

Bad:

interface Employee
{
    public function work(): void;
    public function eat(): void;
}

final readonly class HumanEmployee implements Employee
{
    public function work(): void
    {
        // ....working
    }

    public function eat(): void
    {
        // ...... eating in lunch break
    }
}

final readonly class RobotEmployee implements Employee
{
    public function work(): void
    {
        //.... working much more
    }

    public function eat(): void
    {
        //.... robot can't eat, but it must implement this method
    }
}

Good:

Not every worker is an employee, but every employee is a worker.

interface Workable
{
    public function work(): void;
}

interface Feedable
{
    public function eat(): void;
}

interface Employee extends Feedable, Workable
{
}

final readonly class HumanEmployee implements Employee
{
    public function work(): void
    {
        // ....working
    }

    public function eat(): void
    {
        //.... eating in lunch break
    }
}

// robot can only work
final readonly class RobotEmployee implements Workable
{
    public function work(): void
    {
        // ....working
    }
}

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Dependency Inversion Principle (DIP)

This principle states two essential things:

High-level modules should not depend on low-level modules. Both should depend on abstractions.
Abstractions should not depend upon details. Details should depend on abstractions.

This can be hard to understand at first, but if you've worked with PHP frameworks (like Symfony), you've seen an implementation of this principle in the form of Dependency Injection (DI). While they are not identical concepts, DIP keeps high-level modules from knowing the details of its low-level modules and setting them up. It can accomplish this through DI. A huge benefit of this is that it reduces the coupling between modules. Coupling is a very bad development pattern because it makes your code hard to refactor.

Bad:

class Employee
{
    public function work(): void
    {
        // ....working
    }
}

class Robot extends Employee
{
    public function work(): void
    {
        //.... working much more
    }
}

class Manager
{
    public function __construct(
        private Employee $employee
    ) {}

    public function manage(): void
    {
        $this->employee->work();
    }
}

Good:

interface Employee
{
    public function work(): void;
}

final readonly class Human implements Employee
{
    public function work(): void
    {
        // ....working
    }
}

final readonly class Robot implements Employee
{
    public function work(): void
    {
        //.... working much more
    }
}

final readonly class Manager
{
    public function __construct(
        private Employee $employee
    ) {}

    public function manage(): void
    {
        $this->employee->work();
    }
}

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Don't repeat yourself (DRY)

Try to observe the DRY principle.

Do your absolute best to avoid duplicate code. Duplicate code is bad because it means that there's more than one place to alter something if you need to change some logic.

Imagine if you run a restaurant and you keep track of your inventory: all your tomatoes, onions, garlic, spices, etc. If you have multiple lists that you keep this on, then all have to be updated when you serve a dish with tomatoes in them. If you only have one list, there's only one place to update!

Often you have duplicate code because you have two or more slightly different things, that share a lot in common, but their differences force you to have two or more separate functions that do much of the same things. Removing duplicate code means creating an abstraction that can handle this set of different things with just one function/module/class.

Getting the abstraction right is critical, that's why you should follow the SOLID principles laid out in the Classes section. Bad abstractions can be worse than duplicate code, so be careful! Having said this, if you can make a good abstraction, do it! Don't repeat yourself, otherwise you'll find yourself updating multiple places any time you want to change one thing.

Bad:

function showDeveloperList(array $developers): void
{
    foreach ($developers as $developer) {
        $expectedSalary = $developer->calculateExpectedSalary();
        $experience = $developer->getExperience();
        $githubLink = $developer->getGithubLink();

        render([$expectedSalary, $experience, $githubLink]);
    }
}

function showManagerList(array $managers): void
{
    foreach ($managers as $manager) {
        $expectedSalary = $manager->calculateExpectedSalary();
        $experience = $manager->getExperience();
        $githubLink = $manager->getGithubLink();

        render([$expectedSalary, $experience, $githubLink]);
    }
}

Good:

interface Employee
{
    public function calculateExpectedSalary(): int;
    public function getExperience(): int;
    public function getGithubLink(): string;
}

function showEmployeeList(array $employees): void
{
    foreach ($employees as $employee) {
        render([
            $employee->calculateExpectedSalary(),
            $employee->getExperience(),
            $employee->getGithubLink()
        ]);
    }
}

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Modern PHP Features

Union Types

PHP 8.0 introduced union types, allowing a value to be one of several types.

Good:

function processId(int|string $id): string
{
    return match(true) {
        is_int($id) => "Processing ID: {$id}",
        is_string($id) => "Processing string ID: {$id}",
    };
}

class ApiResponse
{
    public function __construct(
        public array|object $data,
        public int|null $errorCode = null
    ) {}
}

Named Arguments

PHP 8.0 introduced named arguments, making function calls more readable.

Good:

readonly class User
{
    public function __construct(
        public string $name,
        public string $email,
        public int $age,
        public bool $isActive = true,
        public ?string $department = null
    ) {}
}

// Clear and readable
$user = new User(
    name: 'John Doe',
    email: 'john@example.com',
    age: 30,
    department: 'Engineering'
);

Attributes

PHP 8.0 introduced attributes as a structured form of metadata.

Good:

#[Attribute]
readonly class Route
{
    public function __construct(
        public string $path,
        public string $method = 'GET'
    ) {}
}

#[Attribute]
readonly class Validate
{
    public function __construct(
        public string $rule
    ) {}
}

class UserController
{
    #[Route('/users', 'POST')]
    public function createUser(
        #[Validate('required|email')] string $email,
        #[Validate('required|min:3')] string $name
    ): User {
        // ...
    }
}

First-class Callable Syntax

PHP 8.1 introduced first-class callable syntax for creating closures from callable.

Good:

$users = [
    new User('Alice', 'alice@example.com', 25),
    new User('Bob', 'bob@example.com', 30),
    new User('Charlie', 'charlie@example.com', 35),
];

// First-class callable syntax
$names = array_map($users->getName(...), $users);
$emails = array_map($users->getEmail(...), $users);

// Or for static methods
$validated = array_filter($emails, EmailValidator::isValid(...));

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