Introduction: Why PHP Interfaces Matter

In modern object-oriented programming, flexibility and consistency are key. As your PHP applications grow in complexity, you'll find yourself needing a way to define rules or contracts that multiple classes must follow.

That's exactly what interfaces are for in PHP.

They help enforce a structure without dictating how functionality should be implemented. Interfaces are especially useful for:

• Dependency injection

• Service abstraction

• Code testability

• Plugin systems and design patterns

Let’s dive into what interfaces are, why you need them, and how to use them properly.

What Is an Interface in PHP?

An interface is a contract that defines method signatures without providing their implementation. Any class that implements an interface must define all the methods declared in that interface.

Key Characteristics:

• Declared using the interface keyword.

• Contains method declarations only (no logic).

• Classes use the implements keyword to adopt the interface.

• A class can implement multiple interfaces (unlike abstract classes).

Defining and Implementing Interfaces in PHP

✅ Defining an Interface

interface Logger {
    public function log($message);
}

✅ Implementing an Interface

class FileLogger implements Logger {
    public function log($message) {
        echo "Logging to file: $message\n";
    }
}

Attempting to skip implementing log() would throw a fatal error.

Interface vs Abstract Class in PHP

Full Example: PHP Interface in Action

<?php
// Define an interface
interface PaymentGateway {
    public function pay($amount);
}

// Implement the interface
class PayPal implements PaymentGateway {
    public function pay($amount) {
        echo "Processing $$amount payment via PayPal...\n";
    }
}

class Stripe implements PaymentGateway {
    public function pay($amount) {
        echo "Processing $$amount payment via Stripe...\n";
    }
}

// Use interface-based dependency injection
function processPayment(PaymentGateway $gateway, $amount) {
    $gateway->pay($amount);
}

// Example usage
processPayment(new PayPal(), 100);  // Output: Processing $100 payment via PayPal...
processPayment(new Stripe(), 200);  // Output: Processing $200 payment via Stripe...
?>

This is a real-world use case of interfaces: switching implementations without changing your logic.

Tips & Common Pitfalls

✅ Best Practices

• Use interfaces to decouple code and make it more testable.

• Use type-hinting with interfaces for flexible method arguments.

• Combine multiple interfaces for modular design.

❌ Common Mistakes

• Forgetting to implement all methods from the interface.

• Confusing abstract classes with interfaces (they serve different purposes).

• Adding property definitions in interfaces before PHP 7.4 (will throw errors).

Advanced Use: Multiple Interfaces

interface Flyable {
    public function fly();
}

interface Swimmable {
    public function swim();
}

class Duck implements Flyable, Swimmable {
    public function fly() {
        echo "Duck is flying\n";
    }

    public function swim() {
        echo "Duck is swimming\n";
    }
}

This allows you to compose behaviors cleanly using multiple interfaces.

When Should You Use Interfaces?

• When defining shared behaviors for unrelated classes.

• When designing for dependency injection.

• When following SOLID principles, especially the Interface Segregation Principle.

• When building plugin systems, middlewares, or event handlers.

Conclusion: Code for the Future with Interfaces

PHP interfaces are a core part of object-oriented design. They allow your code to remain flexible, testable, and maintainable, especially in large applications and modern frameworks.

✅ Key Takeaways

• Interfaces define what a class must do, not how.

• They help build loosely coupled and easily testable code.

• You can implement multiple interfaces to compose behaviors.

Interfaces are your friend when writing clean, professional-grade PHP.