Class Inheritance and Polymorphism in PHP

Last updated 23-07-23 04:44

Introduction to Class Inheritance

Class inheritance is a fundamental concept in object-oriented programming. It allows the creation of new classes (subclasses) based on existing classes (superclasses). The subclass inherits the properties and methods of the superclass, enabling code reuse and promoting a modular and organized code structure.

Creating a Subclass

To create a subclass in PHP, we use the extends keyword followed by the name of the superclass. Let's consider an example where we have a superclass called Vehicle and a subclass called Car:

class Vehicle {
   // superclass code here

class Car extends Vehicle {
   // subclass code here

Inheriting Properties and Methods

When a subclass extends a superclass, it inherits all the properties and methods defined in the superclass. This means that the subclass can access and use those inherited properties and methods as if they were defined within the subclass itself.

class Vehicle {
   protected $brand;

   public function __construct($brand) {
      $this->brand = $brand;

   public function startEngine() {
      echo "Starting the engine of a {$this->brand} vehicle.";

class Car extends Vehicle {
   // additional subclass code here

Overriding Methods

In some cases, a subclass may want to provide a different implementation for a method inherited from its superclass. This is known as method overriding. To override a method, we simply redefine it in the subclass with the same name and modify its behavior as required.

class Vehicle {
   public function startEngine() {
      echo "Starting the engine of a vehicle.";

class Car extends Vehicle {
   public function startEngine() {
      echo "Starting the engine of a car.";

$car = new Car();
$car->startEngine(); // Output: Starting the engine of a car.

Access Modifiers and Inheritance

Access modifiers, such as public, protected, and private, play an important role in inheritance. The visibility of properties and methods in the superclass determines their accessibility in the subclass.

  • public: Public properties and methods are accessible in both the superclass and subclass.
  • protected: Protected properties and methods are accessible within the class itself and its subclasses but not from outside.
  • private: Private properties and methods are only accessible within the class that defines them and not in any subclasses.

Introduction to Polymorphism

Polymorphism allows objects of different classes to be treated as objects of a common superclass. It enables code to work with objects dynamically, based on their common interface, rather than their specific class.

Implementing Polymorphism in PHP

In PHP, polymorphism can be achieved through method overriding and interface implementation. By defining common methods in interfaces or superclasses, different classes can implement those methods according to their specific behaviors.

interface Shape {
   public function calculateArea();

class Circle implements Shape {
   public function calculateArea() {
      // Implementation for calculating the area of a circle

class Rectangle implements Shape {
   public function calculateArea() {
      // Implementation for calculating the area of a rectangle

$circle = new Circle();
$rectangle = new Rectangle();

$circle->calculateArea();    // Calculate area of the circle
$rectangle->calculateArea(); // Calculate area of the rectangle

Abstract Classes and Interfaces

Abstract classes and interfaces provide additional tools for implementing class inheritance and polymorphism in PHP.

  • Abstract classes: They define common methods and properties that can be shared among multiple subclasses. Abstract classes cannot be instantiated and require subclasses to provide implementations for their abstract methods.
  • Interfaces: They define a contract that classes must adhere to. They specify method signatures without providing implementations. Classes can implement multiple interfaces, enabling them to define a common set of methods from multiple sources.

Polymorphism with Abstract Classes and Interfaces

By using abstract classes and interfaces, we can achieve a higher degree of polymorphism in PHP. Abstract classes provide a way to define common behaviors, while interfaces allow for the implementation of multiple contracts.

abstract class Animal {
   abstract public function makeSound();

interface CanFly {
   public function fly();

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

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

   public function fly() {
      echo "Flying...";

Benefits of Class Inheritance and Polymorphism

Class inheritance and polymorphism offer several advantages:

  1. Code reusability: Inherited properties and methods can be reused in subclasses, reducing redundant code.
  2. Modularity: Inheritance promotes a modular code structure, making code easier to understand and maintain.
  3. Extensibility: Subclasses can add new functionality to inherited methods or override them to provide custom behavior.
  4. Flexibility: Polymorphism allows objects of different classes to be treated uniformly, enhancing code flexibility and scalability.

Best Practices for Using Class Inheritance and Polymorphism

To make the most of class inheritance and polymorphism, consider the following best practices:

  • Follow the Single Responsibility Principle (SRP): Ensure that each class has a single responsibility to avoid overly complex hierarchies.
  • Keep Inheritance Hierarchies Short: Avoid deep inheritance hierarchies, as they can lead to maintenance issues and reduce code clarity.
  • Favor Composition over Inheritance: Use composition to combine the behavior of multiple classes rather than relying solely on inheritance.
  • Use Interfaces Wisely: Choose interfaces wisely to provide a clear and consistent contract for implementing classes.

Common Mistakes to Avoid

While working with class inheritance and polymorphism, be cautious of the following mistakes:

  • Overusing Inheritance: Overusing inheritance can lead to inflexible and tightly coupled code.
  • Violating Encapsulation: Avoid accessing private members of a superclass directly from a subclass.
  • Ignoring Interface Segregation: Create small, focused interfaces to prevent classes from implementing unnecessary methods.


Class inheritance and polymorphism are essential concepts in PHP's object-oriented paradigm. They enable developers to build flexible, maintainable, and scalable applications by promoting code reusability and modularity. By leveraging inheritance and polymorphism effectively, developers can create robust solutions that adapt to changing requirements.


Q: What is the difference between class inheritance and interface implementation?

A: Class inheritance allows a subclass to inherit properties and methods from a superclass, promoting code reuse. Interface implementation, on the other hand, defines a contract that classes must adhere to, specifying method signatures without providing implementations.

Q: Can a class implement multiple interfaces in PHP?

A: Yes, a class can implement multiple interfaces in PHP. This allows the class to define the implementation for all the methods specified in those interfaces, providing a way to achieve multiple contracts.

Q: What is the role of abstract classes in polymorphism?

A: Abstract classes play a crucial role in defining common behaviors and contracts for subclasses. They can have abstract methods that must be implemented by their subclasses, promoting a higher degree of polymorphism.

Q: How does polymorphism enhance code flexibility?

A: Polymorphism allows different objects to be treated uniformly through their common interface. This enhances code flexibility as the code can work with different objects without needing to know their specific classes.

Q: When to favor composition over inheritance?

A: Favor composition over inheritance when you want to combine the behavior of multiple classes without creating deep inheritance hierarchies. Composition allows for more flexibility and promotes code modularity.

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