Interface vs Abstract Class

Interface

An interface is an OOP element that groups a set of function declarations without implementing them. It specifies the contract (the "what") that a class must fulfill.

Rules

• Cannot contain any implementation of methods.
• All methods must be public.
• Cannot contain variables/properties (but can have constants).
• A class can implement multiple interfaces.

interface Animal {
    public function makeSound();
}

class Dog implements Animal {
    public function makeSound() {
        echo "Bark";
    }
}

Abstract Class

An abstract class is like a partially built class. It can contain both abstract methods (which must be implemented by children) and regular methods (which provide default behavior).

Rules

• Can contain both abstract and concrete methods.
• Methods can be public, protected, or private (abstract methods can't be private).
• Can contain properties (variables).
• A class can extend only one abstract class.
• You cannot instantiate an abstract class.

abstract class Animal {
    public $name;

    // Abstract method: no body
    abstract public function makeSound();

    // Concrete method: has body
    public function sleep() {
        echo "Sleeping...";
    }
}

Comparison Table

Feature	Interface	Abstract Class
Inheritance	A class can implement multiple interfaces.	A class can extend only one abstract class.
Methods	Only method signatures, no body.	Can have both abstract and concrete methods.
Properties	No properties (constants only).	Can have properties.
Visibility	All methods must be public.	Methods can be public, protected.
Purpose	To define a contract (behavior).	To provide a base for a group of classes.