PHP Interfaces

An interface defines a contract — method signatures a class must implement. A class can implement multiple interfaces.

1 - Defining and Implementing

interface Drawable {
    public function draw(): string;
}

interface Serializable {
    public function serialize(): string;
    public function unserialize(string $data): void;
}

class Circle implements Drawable, Serializable {
    public function __construct(private float $radius) {}

    public function draw(): string {
        return "Drawing circle r={$this->radius}";
    }

    public function serialize(): string {
        return json_encode(["radius" => $this->radius]);
    }

    public function unserialize(string $data): void {
        $this->radius = json_decode($data, true)["radius"];
    }
}

$c = new Circle(5);
echo $c->draw();      // Drawing circle r=5
echo $c->serialize(); // {"radius":5}

2 - Interface Constants

interface HttpStatus {
    const OK        = 200;
    const NOT_FOUND = 404;
    const ERROR     = 500;
}

echo HttpStatus::OK; // 200

3 - Type-hinting with Interfaces

function render(Drawable $shape): void {
    echo $shape->draw();
}

// Accepts any class that implements Drawable
render(new Circle(3));
render(new Rectangle(5, 2));