PHP Interfaces

Interfaces & Polymorphism (interface, implements)

An interface defines a strict contract of public methods that implementing classes must fulfill. Unlike abstract classes, a class can implement multiple interfaces, enabling true polymorphism and decoupled dependency injection.

Abstract Class vs Interface

Interface Architecture

flowchart TD
    A["interface LoggerInterface"] --> B["FileLogger implements LoggerInterface"]
    A --> C["DatabaseLogger implements LoggerInterface"]
    A --> D["CloudLogger implements LoggerInterface"]
    E["Application Service"] -->|Type-hints contract| A

Practical Code Example

<?php
declare(strict_types=1);

interface NotifierInterface
{
    public function sendNotification(string $recipient, string $message): bool;
}

interface LoggableInterface
{
    public function getLogHeader(): string;
}

// Class implementing multiple interfaces
class EmailNotifier implements NotifierInterface, LoggableInterface
{
    public function sendNotification(string $recipient, string $message): bool
    {
        echo "Sending Email to '$recipient': $message
";
        return true;
    }

    public function getLogHeader(): string
    {
        return "[EMAIL_SERVICE]";
    }
}

// Polymorphic Service dependent on Interface Contract
class UserRegistrationService
{
    public function __construct(private NotifierInterface $notifier) {}

    public function registerUser(string $email): void
    {
        // Business registration logic...
        $this->notifier->sendNotification($email, "Welcome to our platform!");
    }
}

$service = new UserRegistrationService(new EmailNotifier());
$service->registerUser("[email protected]");

Best Practices

• Type-Hint Interfaces Over Concrete Classes: Always type-hint interface contracts in constructors to allow easy swapping of implementation services.
• Keep Interfaces Small & Focused (Interface Segregation Principle): Prefer multiple small specialized interfaces (Readable, Writable) over giant monolithic interfaces.
• Use implements for Contracts: Enable flexible mocking when writing automated unit tests.

Self-Check Challenge

Write an interface CacheInterface with a method signature public function get(string $key): mixed;!