What are Abstract Classes in PHP?

In PHP, abstract classes are classes that cannot be instantiated on their own and are meant to be extended by other classes. They are used as blueprints for other classes, enforcing a consistent interface while allowing flexibility in implementation.

An abstract class is defined using the abstract keyword. It can include both abstract methods (methods without a body) and concrete methods (methods with a full implementation). Abstract methods must be implemented by any subclass that extends the abstract class.

Syntax Example:

abstract class Animal {
    abstract public function makeSound();

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

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

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

Key Characteristics:

• You cannot create an object of an abstract class directly.
• Abstract methods act as placeholders for functionality that must be defined in child classes.
• Abstract classes can have properties, constructors, and concrete methods, just like regular classes.

Use Cases:

• Creating a common interface for a group of related classes while deferring specific implementations to child classes.
• Enforcing certain methods across all subclasses (e.g., render() in a base View class).
• Organizing code to follow the Template Method design pattern.

Abstract Classes vs Interfaces:

• Abstract classes can contain both implemented and unimplemented methods, while interfaces can only declare method signatures (until PHP 8.1, which introduced default methods in interfaces).
• A class can extend only one abstract class but can implement multiple interfaces.

In summary, abstract classes provide a flexible yet structured way to define base functionality and ensure consistency across subclasses in object-oriented PHP.