Can you list the key advantages and features of abstract classes in object-oriented programming?

Abstract classes in object-oriented programming possess the following key advantages and characteristics:

• Cannot instantiate: An abstract class serves as a template and cannot be instantiated directly.
• Interface definition: Allows declaring abstract methods that must be implemented in subclasses.
• Providing common functionality: Can contain both abstract and concrete methods, offering a base implementation for inheritors.
• Polymorphism support: Enables working with subclass objects through a reference to the abstract class.

Example in PHP:

abstract class Animal {
    abstract public function makeSound();

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

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

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

Thus, abstract classes help structure code by ensuring the implementation of key methods in subclasses and reducing code duplication.