Basic Inheritance

Part of the Object Oriented Programming section of Coddy's PHP journey — lesson 21 of 91.

Inheritance allows a class to acquire properties and methods from another class. Instead of rewriting the same code, you can create a base class with shared functionality and have other classes inherit from it.

The class being inherited from is called the parent class (or base class), and the class that inherits is called the child class. You use the extends keyword to establish this relationship:

<?php
class Animal {
    public $name;
    
    public function __construct($name) {
        this->name = $name;
    }
    
    public function speak() {
        return "$this->name makes a sound";
    }
}

class Dog extends Animal {
    public function fetch() {
        return "$this->name fetches the ball";
    }
}

$dog = new Dog("Buddy");
echo $dog->speak() . "\n";
echo $dog->fetch();

Output:

Buddy makes a sound
Buddy fetches the ball

The Dog class automatically has access to the $name property, the constructor, and the speak() method from Animal. It also defines its own fetch() method that only dogs have.

This creates a clear hierarchy: every dog is an animal, so it makes sense for Dog to inherit from Animal. The child class can use everything from the parent while adding its own specialized behavior.

Key Point: Inheritance promotes code reuse and establishes logical relationships between classes. The child class inherits all public and protected members from the parent, reducing duplication and making your code easier to maintain.

Challenge

Easy

Let's build a vehicle hierarchy system that demonstrates how inheritance allows child classes to share functionality from a parent class while adding their own specialized behaviors.

You'll organize your code across three files:

Vehicle.php — Define a Vehicle class that serves as the parent for all vehicles. It should have a public $brand property and a constructor that accepts and sets the brand. Include a start() method that returns "[brand] is starting" and a stop() method that returns "[brand] is stopping".
Motorcycle.php — Define a Motorcycle class that extends Vehicle. Include the Vehicle file at the top. The Motorcycle class doesn't need its own constructor since it will inherit the parent's constructor. Add a wheelie() method that returns "[brand] is doing a wheelie!" — this is a behavior unique to motorcycles.
main.php — Include the Motorcycle file (which already includes Vehicle). You'll receive one input: the motorcycle brand. Create a Motorcycle object with this brand. Print the result of calling start(), then wheelie(), then stop(), each on its own line.

Notice how the Motorcycle class automatically inherits the $brand property, the constructor, and both the start() and stop() methods from Vehicle. The child class only needs to define what makes it unique — in this case, the ability to do a wheelie. This is the power of inheritance: write common functionality once in the parent, and let children reuse it while adding their own specializations.

Cheat sheet

Inheritance allows a class to acquire properties and methods from another class using the extends keyword. The class being inherited from is called the parent class (or base class), and the class that inherits is called the child class.

<?php
class Animal {
    public $name;
    
    public function __construct($name) {
        $this->name = $name;
    }
    
    public function speak() {
        return "$this->name makes a sound";
    }
}

class Dog extends Animal {
    public function fetch() {
        return "$this->name fetches the ball";
    }
}

$dog = new Dog("Buddy");
echo $dog->speak(); // Buddy makes a sound
echo $dog->fetch(); // Buddy fetches the ball

The child class inherits all public and protected members from the parent, including properties, constructors, and methods. The child can also add its own specialized methods and properties.

Inheritance promotes code reuse and establishes logical relationships between classes, reducing duplication and making code easier to maintain.

Try it yourself

<?php
// Include the Motorcycle class (which already includes Vehicle)
require_once 'Motorcycle.php';

// Read input
$brand = trim(fgets(STDIN));

// TODO: Create a Motorcycle object with the brand

// TODO: Print the result of start()

// TODO: Print the result of wheelie()

// TODO: Print the result of stop()
?>

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

Basic Inheritance

Cheat sheet

All lessons in Object Oriented Programming