If you've ever dived deep into PHP object-oriented programming, you've probably come across abstract classes and interfaces. These are two powerful features that help you define structure in your code. But let's be real, if you're just starting, they can be downright confusing. When should you use an abstract class? When is an interface a better option? And how do they actually work?

In this guide, we'll break it all down in a simple, beginner-friendly way. By the end, you’ll know exactly when to use an abstract class, when to go for an interface, and how to implement them effectively.

What Are Abstract Classes in PHP?

Think of an abstract class like a blueprint for other classes. It’s like a semi-built car chassis, some parts are already defined, but other parts need to be completed before the car can hit the road.

Key Features of Abstract Classes:

  • You cannot create an object of an abstract class directly.

  • They can have both abstract methods (methods without a body) and regular methods (fully implemented functions).

  • Abstract classes can define properties (variables) that child classes inherit.

  • They help you define shared behavior across multiple related classes.

Example of an Abstract Class:

abstract class Animal {

    protected $name;

    public function __construct($name) {

        $this->name = $name;

    }

    abstract public function makeSound(); // Abstract method (must be defined in child class)

    public function sleep() {

        return "$this->name is sleeping...";

    }

}

Now, if you try to instantiate Animal, PHP will throw an error because abstract classes must be extended.

$dog = new Animal("Dog"); // ❌ Fatal Error

Instead, you create a child class that extends Animal and implements the missing method:

class Dog extends Animal {

    public function makeSound() {

        return "Woof! Woof!";

    }

}

$dog = new Dog("Buddy");

echo $dog->makeSound(); // ✅ Output: Woof! Woof!

echo $dog->sleep(); // ✅ Output: Buddy is sleeping...

What Are Interfaces in PHP?

While abstract classes are like blueprints, PHP interfaces are more like contracts. They say, “Hey, any class that implements me must follow these rules.” Unlike abstract classes, interfaces only define method signatures, they do not contain any actual method implementations.

Key Features of Interfaces:

  • You cannot have properties in an interface.

  • All methods must be public and abstract (no body allowed).

  • A class can implement multiple interfaces (but can only extend one abstract class).

Example of an Interface:

interface CanFly {

    public function fly(); // No implementation, just a rule

}

class Bird implements CanFly {

    public function fly() {

        return "Flapping wings and flying high!";

    }

}

$eagle = new Bird();

echo $eagle->fly(); // ✅ Output: Flapping wings and flying high!

The biggest advantage of interfaces? They allow multiple inheritance. A class can implement multiple interfaces but extend only one abstract class.

Abstract Classes vs Interfaces: Key Differences

Feature

Abstract Class

Interface

Can have properties?

✅ Yes

❌ No

Can have method implementations?

✅ Yes

❌ No

Can be instantiated?

❌ No

❌ No

Can extend multiple?

❌ No

✅ Yes (can implement multiple)

Use case

When multiple classes share some behavior

When enforcing structure without shared behavior

Real-World Analogy

Think of an abstract class as a template for smartphones, they all have screens, batteries, and cameras, but different manufacturers can add their own unique features.

On the other hand, an interface is like a contract for chargers, whether it's an iPhone, Samsung, or OnePlus, they all must support USB-C or Lightning charging to work.

When to Use Abstract Classes vs. Interfaces?

✅ Use an abstract class when:

  • You need shared code (some methods already implemented).

  • The classes are closely related (e.g., all animals have a sleep function).

  • You want to define properties that will be inherited by child classes.

✅ Use an interface when:

  • You need to enforce specific behavior across unrelated classes.

  • You require multiple inheritance (since PHP only allows one parent class but multiple interfaces).

  • You’re designing a flexible system where different classes can follow the same contract.

Common Mistakes and Misconceptions

???? Mistake #1: Trying to instantiate an abstract class

$animal = new Animal(); // ❌ Fatal error

Fix: Extend the abstract class first.

???? Mistake #2: Adding properties to an interface

interface Vehicle {

    public $speed; // ❌ Error: Interfaces cannot have properties

}

Fix: Define the property in a class instead.

???? Mistake #3: Using abstract classes when interfaces would be better If your abstract class only has abstract methods and no shared behavior, an interface is a better choice.

Conclusion

Both abstract classes and interfaces have their place in PHP. Abstract classes allow you to share functionality among related classes, while interfaces enforce a contract for multiple, unrelated classes. Choosing between them depends on your specific use case. If you need to share behavior, go with an abstract class. If you need flexibility and multiple inheritance, interfaces are your best bet!

That’s a wrap! ???? Hope this clears up the confusion between abstract classes and interfaces. Got any questions? Drop them in the comments! ???? Want to learn concept of PHP? Read this