server.php

<?php
session_start();

// initializing variables
$username = "";
$email = "";
$errors = [];
$id = "";
$db = mysqli_connect("localhost", "root", "", "project");

// REGISTER USER
if (isset($_POST["reg_user"])) {
    $id = mysqli_real_escape_string($db, $_POST["id"]);
    $username = mysqli_real_escape_string($db, $_POST["username"]);
    $email = mysqli_real_escape_string($db, $_POST["email"]);
    $password_1 = mysqli_real_escape_string($db, $_POST["password_1"]);
    $password_2 = mysqli_real_escape_string($db, $_POST["password_2"]);

    // form validation: ensure that the form is correctly filled ...
    // by adding (array_push()) corresponding error unto $errors array
    if (empty($id)) {
        array_push($errors, "ID is required");
    }
    if (empty($username)) {
        array_push($errors, "Username is required");
    }
    if (empty($email)) {
        array_push($errors, "Email is required");
    }
    if (empty($password_1)) {
        array_push($errors, "Password is required");
    }
    if ($password_1 != $password_2) {
        array_push($errors, "The two passwords do not match");
    }

    // first check the database to make sure
    // a user does not already exist with the same username and/or email
    $user_check_query = "SELECT * FROM users WHERE username='$username' OR email='$email' LIMIT 1";
    $result = mysqli_query($db, $user_check_query);
    $user = mysqli_fetch_assoc($result);

    if ($user) {
        // if user exists
        if ($user["username"] === $username) {
            array_push($errors, "Username already exists");
        }

        if ($user["email"] === $email) {
            array_push($errors, "email already exists");
        }
    }

    // Finally, register user if there are no errors in the form
    if (count($errors) == 0) {
        $password = $password_1; //encrypt the password before saving in the database

        $query = "INSERT INTO users (id, username, email, password) 
  			  VALUES('$id', '$username', '$email', '$password')";

        mysqli_query($db, $query);
        // $_SESSION['username'] = $username;
        // $_SESSION['success'] = "You are now logged in";
        header("location: index.php");
    }
}

// LOGIN USER
if (isset($_POST["login_user"])) {
    $username = mysqli_real_escape_string($db, $_POST["username"]);
    $password = mysqli_real_escape_string($db, $_POST["password"]);
    if (empty($username)) {
        array_push($errors, "Username is required");
    }
    if (empty($password)) {
        array_push($errors, "Password is required");
    }

    if (count($errors) == 0) {
        $password = $password;
        $query = "SELECT * FROM users WHERE username='$username' AND password='$password'";
        $results = mysqli_query($db, $query);
        if (mysqli_num_rows($results) == 1) {
            $_SESSION["username"] = $username;
            $_SESSION["success"] = "You are now logged in";
            header("location: index.php");
        } else {
            array_push($errors, "Wrong username/password combination");
        }
    }
}

?>