Interfaces in PHP (Flexible, Testable Design)

Use interfaces to define contracts, enabling clean architecture and easy mocking in tests.

An interface defines a contract: “any class that implements this must provide these methods.”

This is powerful for: - clean architecture - swapping implementations - unit testing (mocking)

Example: PaymentGateway interface

```php <?php interface PaymentGateway { public function charge(float $amount): bool; }

class StripeGateway implements PaymentGateway { public function charge(float $amount): bool { return true; } }

class CheckoutService { public function __construct(private PaymentGateway $gateway) {}

public function pay(float $amount): bool { return $this->gateway->charge($amount); } }

$checkout = new CheckoutService(new StripeGateway()); $checkout->pay(50); ?> ```

CheckoutService does not care if you use Stripe, PayPal, or something else. That means your code is easier to maintain.

> Next: Namespaces, avoid class name conflicts in real projects.