PHP Interfaces: Define Contracts for Classes with Examples

<?php
  interface InterfaceName {
    public function someMethod1();
    public function someMethod2($name, $color);
    public function someMethod3() : string;
  }
?>

PHP - Interfaces vs. Abstract Classes

Both interfaces and abstract classes are used to define blueprints for other classes, but they have key differences in structure, usage, and flexibility.

Feature	Interface	Untitle
Definition	Uses `interface` keyword	Uses `abstract class` keyword
Method Implementation	Cannot have method implementations (only method declarations)	Can have both abstract (no implementation) and concrete (implemented) methods
Properties (Variables)	Cannot have properties	Can have properties
Method Access Modifiers	Always `public`	Can be `public`, `protected`, or `private`
Constructors	Cannot have constructors	Can have constructors
Multiple Inheritance	A class can implement multiple interfaces	A class can extend only one abstract class
Use Case	Used when multiple unrelated classes need to implement the same behavior	Used for common functionality among related classes

Example

<?php

interface Animal {

public function makeSound();

}

class Cat implements Animal {

public function makeSound() {

echo "Meow";

}

$animal = new Cat();

$animal->makeSound();

Try it yourself

From the example above, let's say that we would like to write software which manages a group of animals. There are actions that all of the animals can do, but each animal does it in its own way.

Using interfaces, we can write some code which can work for all of the animals even if each animal behaves differently:

Example

<?php

// Define the Animal interface

interface Animal {

public function makeSound();

}

// Implementing the interface in Cat class

class Cat implements Animal {

public function makeSound() {

return "Meow!";

}

// Implementing the interface in Dog class

class Dog implements Animal {

public function makeSound() {

return "Woof!";

}

// Implementing the interface in Mouse class

class Mouse implements Animal {

public function makeSound() {

return "Squeak!";

}

// Creating an array of different animals

$animals = [new Cat(), new Dog(), new Mouse()];

// Loop through the animals and call makeSound() without knowing their type

foreach ($animals as $animal) {

echo $animal->makeSound() . "<br>";

}

Try it yourself

Explanation

1. Define an Interface (Animal)

The Animal interface declares the makeSound() method.
Any class that implements Animal must define makeSound().

2. Implementing Classes (Cat, Dog, Mouse)

Each animal implements Animal and defines makeSound() differently.

3. Polymorphism: Handling Different Animals in a Loop

We store different objects in an array.
The loop calls makeSound() on each object, without needing to check its type.
This is an example of polymorphism—different objects respond differently to the same method.

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