OOP — ইন্টারফেস আর অ্যাবস্ট্র্যাক্ট ক্লাস

<?php
  interface Payable {
    public function pay($amount);
  }

  class CreditCard implements Payable {
    public function pay($amount) {
      echo "Charged $amount to credit card.";
    }
  }

  class Cash implements Payable {
    public function pay($amount) {
      echo "Received $amount in cash.";
    }
  }

  function checkout(Payable $method, $amount) {
    $method->pay($amount); // works with any Payable, regardless of exact class
  }

  checkout(new CreditCard(), 500);
  checkout(new Cash(), 500);
?>

<?php
  abstract class Shape {
    abstract public function area(); // every subclass must define this

    public function describe() { // shared by every subclass, as-is
      return "This shape has an area of " . $this->area();
    }
  }

  class Circle extends Shape {
    public function __construct(private $radius) {}
    public function area() {
      return round(pi() * $this->radius ** 2, 2);
    }
  }

  $circle = new Circle(5);
  echo $circle->describe(); // "This shape has an area of 78.54"
  // new Shape(); // Error — cannot instantiate an abstract class
?>