PHP Abstract Classes

A class in PHP is considered to be abstract if it cannot be constructed on its own and is mostly used as a model for other classes to inherit from. Utilizing the abstract keyword, abstract classes are created. They enable you to provide methods that any child class must implement, but they may also include actual methods (methods with implementations) that can be inherited by child classes.

Here are some use cases and concepts related to PHP abstract classes:

Defining Abstract Methods: Abstract classes can declare abstract methods using the abstract keyword. These methods have no implementation in the abstract class but must be implemented by any concrete child class that extends the abstract class. For example:

abstract class Shape {
    abstract public function area();
}

Forcing Method Implementation: Abstract classes ensure that certain methods are implemented by child classes, providing a level of contract or interface definition. This helps maintain consistency and ensures that specific methods are available in all derived classes.

Partial Implementation: Abstract classes can also have concrete (non-abstract) methods with actual implementations. These methods can be inherited by child classes, which can then override or extend them as needed.

abstract class Animal {
    abstract public function makeSound();

    public function eat() {
        echo "The animal is eating.";
    }
}

Creating a Base Class:
Abstract classes are often used as base classes or superclasses for other classes. They provide a common set of properties and methods that are shared among multiple derived classes.

class Dog extends Animal {
    public function makeSound() {
        echo "The dog barks.";
    }
}

Polymorphism:
Abstract classes facilitate polymorphism, where you can use objects of different derived classes through a common interface (the abstract class). This makes it easier to write code that works with various related objects.

Restricting Instantiation:
Abstract classes cannot be instantiated directly with the new keyword. Attempting to do so will result in a fatal error. They can only be used as base classes for inheritance.

// This will result in an error:
// $shape = new Shape();

Use in Frameworks and Libraries:
Abstract classes are commonly used in PHP frameworks and libraries to provide extensible and customizable components. Frameworks often define abstract classes for controllers, models, and other core components.

Extensibility:
Abstract classes can be extended further by other concrete classes. This allows for a hierarchical structure where you can have multiple levels of inheritance.

In conclusion, abstract classes in PHP are an effective tool for establishing class hierarchies, guaranteeing method execution, and defining a shared interface for related classes. They are a cornerstone of object-oriented programming, and they are regularly applied in PHP applications and frameworks to encourage code reuse and maintainability.