Extending classes in PHP

Introduction

Extending classes lets you create a new class based on an existing one. This helps you reuse code and add new features easily.

When you want a new class to have all features of an existing class plus some extra.

When you want to organize similar things in a family of classes.

When you want to change or add behavior without rewriting code.

When you want to keep your code clean and avoid repetition.

Syntax

PHP

<?php
class ParentClass {
    // properties and methods
}

class ChildClass extends ParentClass {
    // new or changed properties and methods
}
?>

The extends keyword is used to create a child class.

The child class inherits all public and protected properties and methods from the parent.

Examples

This example shows a child class Dog that changes the speak method of the parent Animal.

PHP

<?php
class Animal {
    public function speak() {
        echo "Animal sound\n";
    }
}

class Dog extends Animal {
    public function speak() {
        echo "Bark!\n";
    }
}

$dog = new Dog();
$dog->speak();
?>

The Car class extends Vehicle and adds a new method openDoor.

PHP

<?php
class Vehicle {
    public function start() {
        echo "Vehicle started\n";
    }
}

class Car extends Vehicle {
    public function openDoor() {
        echo "Door opened\n";
    }
}

$car = new Car();
$car->start();
$car->openDoor();
?>

Sample Program

This program shows a Student class extending Person. The student has a name and a grade. It can greet and study.

PHP

<?php
class Person {
    public $name;

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

    public function greet() {
        echo "Hello, my name is {$this->name}.\n";
    }
}

class Student extends Person {
    public $grade;

    public function __construct($name, $grade) {
        parent::__construct($name);
        $this->grade = $grade;
    }

    public function study() {
        echo "{$this->name} is studying for grade {$this->grade}.\n";
    }
}

$student = new Student("Alice", 10);
$student->greet();
$student->study();
?>

Important Notes

Use parent::__construct() to call the parent class constructor inside the child.

Child classes can override parent methods by defining a method with the same name.

Only public and protected members are inherited; private members are not accessible directly.

Summary

Extending classes helps reuse and organize code by creating child classes from parent classes.

Use extends keyword to make a child class inherit from a parent.

Child classes can add new features or change existing ones easily.