Interfaces in PHP (Flexible, Testable Design)

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
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);
?>

Why this is professional

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.