What is the main purpose of creating abstract classes in object-oriented programming?

The main purpose of abstract classes is to define a common template for a group of related classes, specifying the basic structure and behavior but not implementing all details. An abstract class cannot be instantiated directly; it serves as a base for inheritance and requires derived classes to implement certain methods. This helps ensure consistency and avoid code duplication.

Example in PHP:

abstract class Animal {
    abstract public function makeSound();

    public function sleep() {
        echo "Sleeping...";
    }
}

class Dog extends Animal {
    public function makeSound() {
        echo "Woof!";
    }
}

$dog = new Dog();
$dog->makeSound(); // Outputs: Woof!
$dog->sleep();     // Outputs: Sleeping...