Inheritance in PHP Object-Oriented Programming | CodeSignal Learn

Inheritance in PHP Object-Oriented Programming | CodeSignal Learn

Introduction

Hello again! In this lesson, we will explore inheritance in object-oriented programming (OOP) using PHP. Inheritance is a fundamental concept that allows us to share code between classes, thereby enhancing code reusability and efficiency. We'll delve into inheritance by discussing attribute and method inheritance using practical PHP examples. Our lesson's blueprint will cover defining inheritance, examining attribute inheritance, exploring method inheritance, and understanding the usage of the parent class constructor in PHP. Ready? Let's get started!

Defining Inheritance

Inheritance in PHP entails creating a derived class (child class) that inherits properties and methods from a base class (parent class). It's common to encounter situations where classes share common characteristics, making inheritance extremely useful. Here's an example showcasing a base class Vehicle and a derived class Car: PHP<?php // Define the base class 'Vehicle' class Vehicle { protected $color; protected $brand; // Constructor to initialize 'color' and 'brand' attributes public function __construct($color, $brand) { $this->color = $color; $this->brand = $brand; } } // Define the derived class 'Car' class Car extends Vehicle { // Car inherits from Vehicle private $doors; // Constructor to initialize 'color', 'brand', and 'doors' public function __construct($color, $brand, $doors) { parent::__construct($color, $brand); // Call the parent class constructor $this->doors = $doors; } } ?> We will focus primarily on single inheritance in PHP, where one base class is extended by one derived class.

Attribute Inheritance

With attribute inheritance, a derived class inherits the attributes of a base class, allowing it to make use of shared properties. Consider an example with a base class Artist and a derived class Musician: PHP<?php // Define the base class 'Artist' class Artist { protected $name; // Constructor to initialize 'name' attribute public function __construct($name) { $this->name = $name; } } // Define the derived class 'Musician' class Musician extends Artist { // Musician inherits from Artist private $instrument; // Constructor to initialize 'name' and 'instrument' attributes public function __construct($name, $instrument) { parent::__construct($name); // Call the parent constructor $this->instrument = $instrument; } // Method to display the musician's details public function display() { echo "Name: " . $this->name . "\nInstrument: " . $this->instrument . "\n"; } } // Create an instance of 'Musician' and display its details $john = new Musician("John Lennon", "Guitar"); $john->display(); // Name: John Lennon // Instrument: Guitar ?> The Musician class inherits the name attribute from the Artist class and also possesses its unique attribute, instrument.

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Just like attributes, method inheritance allows a derived class to inherit methods from a base class.

In the following example, the Car class inherits the start method from the Vehicle class:

PHP

<?php

// Define the base class 'Vehicle'
class Vehicle {
    protected $brand;

    // Constructor to initialize 'brand' attribute
    public function __construct($brand) {
        $this->brand = $brand;
    }

    // Method to simulate starting the vehicle
    public function start() {
        echo "The " . $this->brand . " is starting.\n";
    }
}

// Define the derived class 'Car'
class Car extends Vehicle { // Car inherits from Vehicle
    // Constructor to initialize 'brand' attribute
    public function __construct($brand) {
        parent::__construct($brand);
    }
}

// Create an instance of 'Car' and call the inherited 'start' method
$my_car = new Car("BMW");
$my_car->start(); // Output: The BMW is starting.
?>

In PHP, the base class constructor can be called within the derived class constructor using the parent::__construct() syntax. This ensures that a derived class can leverage or extend the functionality of a base class without altering it.

For example, when creating a derived class that adds new attributes, invoking the parent constructor ensures proper initialization, as shown below:

PHP

<?php

// Define the base class 'ParentClass'
class ParentClass {
    protected $value;

    // Constructor to initialize 'value' attribute
    public function __construct($value) {
        $this->value = $value;
    }
}

// Define the derived class 'ChildClass'
class ChildClass extends ParentClass { // ChildClass inherits from ParentClass
    private $additional_value;

    // Constructor to initialize 'value' and 'additional_value'
    public function __construct($value, $additional_value) {
        parent::__construct($value);
        $this->additional_value = $additional_value;
    }

    // Method to display the values
    public function display() {
        echo "Value: " . $this->value . "\nAdditional Value: " . $this->additional_value . "\n";
    }
}

// Create an instance of 'ChildClass' and display its values
$child_class = new ChildClass("value", "additional_value");
$child_class->display();
// Value: value
// Additional Value: additional_value
?>

These examples illustrate how calling the base class constructor in PHP promotes modular and efficient inheritance by enabling derived classes to expand upon existing functionalities cleanly.