Inheritance in PHP

Welcome back! Now that you have a solid understanding of classes and objects in PHP, it's time to build on that knowledge by exploring inheritance. Consider it a natural progression in our journey into object-oriented programming (OOP).

Inheritance allows you to create a new class based on an existing class. By using inheritance, you can reuse code, add new features, and make your programs easier to manage and understand. Let's dive in and see what it's all about.

In this lesson, you'll understand how to use inheritance in PHP. We'll cover:

1. What Inheritance Is
2. How to Implement Inheritance in PHP
3. Why Inheritance Is Beneficial

Inheritance is a way to establish a relationship between a new class (child class) and an existing class (parent class). The child class inherits properties and behaviors (methods) from the parent class.

Here’s a simple example:

<?php

// Define the base class Person with name and age attributes
class Person {
    // Declare properties
    private $name;
    private $age;

    // Constructor to initialize name and age
    public function __construct($name, $age) {
        $this->name = $name;
        $this->age = $age;
    }

    // Display method to show name and age
    public function display() {
        echo "Name: " . $this->name . ", Age: " . $this->age . "\n";
    }

    // Greet method to display a greeting message
    public function greet() {
        echo "Hello, there!\n";
    }
}

// Define the derived class Student, inheriting from Person
class Student extends Person {
    // Declare property for major
    private $major;

    // Constructor to initialize name, age of the base class, and the major of the student
    public function __construct($name, $age, $major) {
        // Call parent constructor to initialize base attributes
        parent::__construct($name, $age);
        $this->major = $major;
    }

    // Display method to show name, age using the base class display method, and major of the student
    public function display() {
        $this->greet();
        parent::display();
        echo "Major: " . $this->major . "\n";
    }
}

// Create a Student object and display its details
$student = new Student("Bob", 25, "Computer Science");
$student->display();

?>

In this snippet, the Student class inherits from the Person class. It reuses the name and age attributes and methods from the Person class and adds a new attribute major and a new display method to show the student's major.

When you declare a child class, you specify the parent class it inherits from by using the extends keyword. The child class can then extend or override the functionality of the parent class.

In our example:

• The Person class is the parent class.
• The Student class is the child class, inheriting name and age from the Person class.
• The Student class also adds a new member, major, and overrides the display method to include information about the major.
  • Notice how Student::display() calls parent::display() to reuse the parent class functionality before adding its details.
  • The greet method is also called from the display method to show how the child class can access parent class methods.