Abstract Class vs Interface in PHP

Defining a contract

Sometimes you want to say "any class of this kind must have these methods". PHP offers two tools for this: an abstract class and an interface. Both set rules that other classes must follow, but they differ in how much they provide. Choosing the right one keeps your code well-organised.

Abstract class

An abstract class can have real methods (with code) and abstract methods (just the name). You cannot create an object from it directly — a child class fills in the blanks.

<?php
  abstract class Shape {
      abstract public function area();      // child must define
      public function describe() {          // shared code
          return "Area is " . $this->area();
      }
  }
  class Square extends Shape {
      public $side = 4;
      public function area() { return $this->side * $this->side; }
  }
  echo (new Square())->describe();
?>

Square must define area(), but reuses the finished describe() from the abstract base.

Interface

An interface only lists method names — no code at all. A class implements it and must provide every method.

<?php
  interface Printable {
      public function printMe();
  }
  class Invoice implements Printable {
      public function printMe() { return "Printing invoice"; }
  }
  echo (new Invoice())->printMe();
?>

The interface guarantees Invoice has a printMe() method, but provides no code itself. A class can implement many interfaces.

Key differences

When to use which

Use an abstract class when related classes share code and each must fill in a few methods — like different shapes that all describe themselves but compute area differently. Use an interface when unrelated classes just need to support the same action — like anything that can be "printed" or "saved", with no shared code.

Common mistakes

• Trying to create an object of an abstract class. You must extend it with a child class first.
• Putting code in an interface. Interfaces only declare method names, not bodies.
• Extending several classes. A class extends only one; use interfaces for multiple contracts.
• Not implementing every interface method. A class must define all methods the interface lists.

Summary

• Both set rules other classes must follow.
• An abstract class can share code and has abstract methods to fill in.
• An interface only lists method names, no code.
• Extend one abstract class; implement many interfaces.
• Abstract = shared base; interface = pure contract.

Next, reuse code across classes with PHP traits, or review the basics in PHP OOP.