Inheritance and Abstract Classes

Inheritance lets a class reuse and extend another class's behavior with extends:

class Animal
{
    public function __construct(protected string $name) {}

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

class Dog extends Animal
{
    public function speak(): string
    {
        return "{$this->name} barks."; // overrides the parent method
    }

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

$dog = new Dog('Rex');
echo $dog->speak(); // 'Rex barks.'

Call the parent implementation from a child with parent::speak(), and prevent further overriding of a method or extending of a class with the final keyword.

Abstract classes cannot be instantiated directly — they define a partial contract that subclasses must complete:

abstract class Shape
{
    abstract public function area(): float; // no body — subclasses must implement it

    public function describe(): string
    {
        return 'Area: ' . $this->area();
    }
}

class Circle extends Shape
{
    public function __construct(private float $radius) {}

    public function area(): float
    {
        return pi() * $this->radius ** 2;
    }
}

Abstract classes are useful when you want to share some common implementation across subclasses while still forcing each one to implement specific methods.