How do abstract class and interface differ in object-oriented programming?

The differences between an abstract class and an interface in PHP are as follows:

• Abstract class can contain both abstract methods (without implementation) and methods with implementation, as well as properties.

• Interface only contains method declarations (starting from PHP 8, default implementations can be added, but this is a special case), and does not contain state (properties).

• A class can inherit from only one abstract class, but can implement multiple interfaces.

• An abstract class is used when you need to define a base implementation and a common contract, whereas an interface is used to define only a contract without implementation.

Example:

abstract class Animal {
    abstract public function makeSound();
    public function sleep() {
        echo "Sleeping";
    }
}

interface CanFly {
    public function fly();
}

class Bird extends Animal implements CanFly {
    public function makeSound() {
        echo "Chirp";
    }
    public function fly() {
        echo "Flying";
    }
}

Thus, the abstract class provides a partial implementation, while the interface only defines a contract.