PHP Interfaces

PHP Interfaces: Building Decoupled, Testable Contracts

A PHP interface defines a strict list of public methods that any implementing class promises to provide, without dictating how those methods work internally. Interfaces enable polymorphism and dependency injection, letting you swap a real email sender or payment API for a fake test version without touching caller code.

Defining and Implementing an Interface

Declare an interface with interface and attach it to classes with implements. A single class can implement multiple interfaces separated by commas (implements MailerInterface, LoggableInterface).

classDiagram
    class MailerInterface {
        <<interface>>
        +send(string to, string subject) bool
    }
    class SmtpMailer {
        +send(string to, string subject) bool
    }
    class ArrayTestMailer {
        +send(string to, string subject) bool
    }
    MailerInterface <|.. SmtpMailer : implements
    MailerInterface <|.. ArrayTestMailer : implements

<?php
declare(strict_types=1);

interface MailerInterface
{
    public function send(string $to, string $subject): bool;
}

class SmtpMailer implements MailerInterface
{
    public function send(string $to, string $subject): bool
    {
        echo "SMTP queued email to {$to}: {$subject}" . PHP_EOL;
        return true;
    }
}

function sendWelcomeEmail(MailerInterface $mailer, string $email): void
{
    $mailer->send($email, 'Welcome to KoderSolution');
}

sendWelcomeEmail(new SmtpMailer(), '[email protected]');

Notice how sendWelcomeEmail() type-hints MailerInterface instead of SmtpMailer. Tomorrow you can pass a SendGridMailer or InMemoryTestMailer without changing a single line inside sendWelcomeEmail().

Common Mistakes to Avoid

• Adding instance properties to an interface: PHP interfaces cannot contain mutable properties—only public method signatures and constants.
• Declaring protected or private methods in an interface: Every method in an interface defines a public contract and must be public.
• Creating "fat" interfaces with 15 unrelated methods: Split large interfaces into small focused contracts (Interface Segregation Principle) so classes are never forced to write dummy stub methods.

Frequently Asked Questions

Can one PHP interface extend another interface?

Yes. An interface can extend one or more parent interfaces using extends, such as interface CacheableRepository extends RepositoryInterface.

Can a class extend a parent class AND implement interfaces together?

Yes. Write extends first, followed by implements: class PdfReport extends BaseReport implements Exportable, Printable.

When should I create an interface vs using a concrete class directly?

Create an interface when you expect multiple implementations (for example, Stripe vs PayPal, Redis vs File cache) or want clean mocking in tests.