"Operator"(-ish) Overloading Techniques in PHP

I'll preface this by defining operator overloading for our purposes to be achieving arbitrary behaviour given some language symbol, and one or more variables. I won't distingish whether or not you have control over what value is returned by these "operators": in some cases you will have control over the return value, in some cases you may only control some side effect. Discard items to your hearts content.

Binary/Ternary Operators

• -> • = • Accessor-Assignment

Under some circumstances you may overload the assignment operator. Namely, if your class implements the __set() method which should have the following signature.

public function __set(string $name, $value): void

This is a ternary operator if you manipulate all three values available to you • -> • = •, or binary if you consider the accessed property to be a single value • = •.

If an inaccessible property on your class is attempted to be set (whether that be because of visibility, or non-existance) then PHP will defer to this method if implemented.

This method will only be called when attempting to access via the use of ->, and will not be called if using the scope resolution operator ::.

Note that return values from this function are ignored, so code like

if ($object->var = 1)

will make sense in the written context (will be truthy because 1 is truthy) but you can not change the result of this if statement by returning a falsey value.

• -> • Accessor

Just because its worth mentioning it alongside __set(), you can similarly use __get() with method signature

public function __get(string $name)

when attempting to access inaccesible properties. Unlike before, the return value here is taken into account.

• []= • Array Append

An object that implements the ArrayAccess interface may use

abstract public function offsetSet ($offset, $value): void

to utilise the array append operator. In this case, $offset will have a value of null. It is up to you whether you wish to consider this a binary operator, and the similar version with $offset being non null a ternary one.

•[•]= • Array-Key Assignment

As above, an object that implements the ArrayAccess interface may use

abstract public function offsetSet ($offset, $value): void

There is no restriction on $offset nor $value, unlike when using arrays.

•[•] Array-Key Accessor

An object that implements the ArrayAccess interface may use

abstract public function offsetGet($offset)

There is no restriction on $offset, unlike when using arrays.

Personally I find this to be the most interesting operator so far, because it allows arbitrary values in both slots (though obviously the value in slot one must implement the required interface).

This operator could be designed to be commutative if both values implement the required interface. And could also/instead be polymorphic based on type detection.

n-ary Operators

•(•, •, ...) Call Operator

An object may implement the magic method __invoke(), which will be called if the object itself is called, passing all arguments through.

This is the freest operator you have control over (though probably also the most ambiguous).

Unary Pseudo-Operators

Stringyness

Okay, so this is not an operator (even by the very loose definition of "some language symbol" above) – but its behaviour can be invoked by one (among many things). A class may implement the __toString() method to determine how it should behave when treated like a string. It should have method signature.

public function __toString(): string

You can invoke __toString() by treating an object like a string in one of various ways, e.g.

echo $object;

$object.'';