Interfaces in PHP

Introduction

What is an Interface?

An interface defines what a class must do, but not how it does it.

It is a 100% abstract blueprint.

In simple terms:

Interface forces a contract on a class.

Why Use Interfaces?

Interfaces help you:

• Enforce consistency across classes
• Achieve multiple inheritance
• Write flexible and scalable code
• Reduce tight coupling

Interface Rules in PHP

• Defined using interface keyword
• All methods are public
• Methods cannot have body
• A class implements interface using implements
• A class must implement all methods

Interface Example

<?php
interface PaymentInterface {
    public function pay($amount);
}
?>

Implementing an Interface

<?php
class CardPayment implements PaymentInterface {
    public function pay($amount) {
        echo "Paid ₹$amount using Card";
    }
}
?>

Another Implementation

<?php
class UpiPayment implements PaymentInterface {
    public function pay($amount) {
        echo "Paid ₹$amount using UPI";
    }
}
?>

Using Interface (Polymorphism)

<?php
function makePayment(PaymentInterface $payment, $amount) {
    $payment->pay($amount);
}

makePayment(new CardPayment(), 1000);
makePayment(new UpiPayment(), 500);
?>

✔ Same function
✔ Different behavior
✔ Loose coupling

Multiple Interfaces

A class can implement multiple interfaces.

<?php
interface Login {
    public function login();
}

interface Logout {
    public function logout();
}

class User implements Login, Logout {
    public function login() {
        echo "User logged in";
    }

    public function logout() {
        echo "User logged out";
    }
}
?>

Interface vs Abstract Class

Real-World Example

• Interface → Logger
• Classes → FileLogger, DatabaseLogger

Switch logging system without changing core code.

When to Use Interface?

✔ When behavior matters, not implementation
✔ When multiple inheritance is needed
✔ When designing APIs or frameworks

Rules of Interfaces in PHP

There are some important rules when working with interfaces:

• All methods inside an interface must be public
• Interfaces cannot contain method bodies
• A class must implement all methods defined in the interface
• Interfaces cannot be instantiated directly
• A class can implement multiple interfaces

Common Mistakes

❌ Trying to add method body
❌ Forgetting to implement all methods
❌ Using interface like a class

Practical Task

• Create an interface called Shape.
  Inside it define the method:
  calculateArea()
• Then create two classes:
  • Circle
  • Rectangle
• Both classes should implement the interface and define the calculateArea() method.
• Create objects of both classes and display their calculated areas.

Summary

• Interfaces define a contract that classes must follow. They help maintain a consistent structure in applications.
• In PHP, interfaces contain only method declarations and do not include method bodies. Any class implementing an interface must provide the implementation for all its methods.
• Interfaces are useful when designing scalable applications and enforcing common behavior across multiple classes.
• They are widely used in frameworks, libraries, and large PHP projects.

In the next tutorial, we’ll learn about Traits in PHP.