PHP Inheritance

Inheritance allows a class to reuse properties and methods from another class. The child class extends the parent class and can override its methods.

extends and parent::

Use extends to inherit from a parent class. Use parent:: to call the parent's constructor or methods from the child.

Basic Inheritance

<?php
class Animal {
    protected string $name;
    protected string $sound;

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

    public function speak(): string {
        return "{$this->name} says {$this->sound}";
    }

    public function describe(): string {
        return "I am {$this->name}";
    }
}

class Dog extends Animal {
    private string $breed;

    public function __construct(string $name, string $breed) {
        parent::__construct($name, "Woof"); // call parent constructor
        $this->breed = $breed;
    }

    // Method overriding
    public function speak(): string {
        return parent::speak() . "! (breed: {$this->breed})";
    }

    public function fetch(): string {
        return "{$this->name} fetches the ball!";
    }
}

$dog = new Dog("Rex", "Labrador");
echo $dog->speak();    // Rex says Woof! (breed: Labrador)
echo $dog->describe(); // I am Rex (inherited)
echo $dog->fetch();    // Rex fetches the ball!

// instanceof operator
echo ($dog instanceof Dog)    ? "Is a Dog\n"    : "";
echo ($dog instanceof Animal) ? "Is an Animal\n" : "";
?>

final Classes and Methods

Mark a class or method as final to prevent it from being extended or overridden.

final Keyword

<?php
class Vehicle {
    public string $type;

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

    // This method cannot be overridden
    final public function getType(): string {
        return $this->type;
    }
}

class Truck extends Vehicle {
    public int $payload;

    public function __construct(int $payload) {
        parent::__construct("Truck");
        $this->payload = $payload;
    }
    // Cannot override getType() — it's final
}

// Final class — cannot be extended
final class Singleton {
    private static ?Singleton $instance = null;

    private function __construct() {}

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

$s1 = Singleton::getInstance();
$s2 = Singleton::getInstance();
var_dump($s1 === $s2); // true — same instance
?>

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PHP Inheritance

extends and parent::

final Classes and Methods

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