php-inheritance

PHP Inheritance is a fundamental concept in object-oriented programming (OOP) within the PHP language. It allows classes to inherit methods, properties, and behaviors from other classes, promoting code reusability and establishing a hierarchical relationship between classes.

Historical Context

Inheritance in PHP was introduced in PHP 4, which marked the language's initial steps towards supporting OOP. However, PHP 4's implementation was limited. The true power of inheritance and OOP in PHP came with PHP 5, released in July 2004. PHP 5 introduced features like:

• Improved PHP Class and object handling.
• Support for interfaces.
• Visibility modifiers (public, protected, private).

These enhancements made PHP's OOP model more robust, closely aligning with other object-oriented languages like Java or C#.

Key Concepts of PHP Inheritance

• Base and Derived Classes: The class from which another class inherits is called the base or parent class, while the class that inherits is called the derived or child class.
• Extending Classes: PHP uses the keyword extends to indicate inheritance. For example:

  class ParentClass {
          // Parent class properties and methods
      }
  
      class ChildClass extends ParentClass {
          // Child class can use parent methods and add new ones
      }

• Method Overriding: A child class can redefine methods from its parent class, customizing or extending their behavior.
• Visibility: PHP provides visibility levels for properties and methods:
  • public: Accessible from anywhere.
  • protected: Accessible within the class itself and by inherited and parent classes.
  • private: Accessible only from within the class itself.
• Constructor Inheritance: Constructors are not automatically inherited; a child class must call the parent constructor if needed using parent::__construct().
• Abstract Classes and Interfaces: PHP supports abstract classes (which can contain abstract methods) and interfaces, which define a contract of methods that implementing classes must provide.

Practical Uses

• Code Reusability: Inheritance reduces code duplication by allowing common functionality to be defined in a parent class.
• Polymorphism: Through method overriding, PHP supports polymorphism, where objects of different classes can be treated as objects of a common superclass.
• Hierarchical Organization: It helps in organizing code in a logical, hierarchical manner, mirroring real-world relationships.

Examples in PHP Frameworks

Many PHP frameworks like Laravel or Symfony extensively use inheritance to create a structured and extensible codebase:

• In Laravel, controllers often extend a base controller class to inherit common functionalities.
• Symfony uses inheritance in its bundle system where bundles can extend and override functionality from parent bundles.

External Resources

• PHP Manual: Inheritance
• PHP The Right Way: Object-Oriented Programming

Related Topics

• php-class
• php-interfaces
• php-abstract-classes
• php-polymorphism