PHP OOP - Classes and Objects

A class is a template for objects, and an object is an instance of class.

OOP Case

Let's assume we have a class named Fruit. A Fruit can have properties like name, color, weight, etc. We can define variables like $name, $color, and $weight to hold the values of these properties.

When the individual objects (apple, banana, etc.) are created, they inherit all the properties and behaviors from the class, but each object will have different values for the properties.

Define a Class

A class is defined by using the class keyword, followed by the name of the class and a pair of curly braces ({}). All its properties and methods go inside the braces:

Syntax

<?php
class Fruit {
// code goes here...
}
?>

Below we declare a class named Fruit consisting of two properties ($name and $color) and two methods set_name() and get_name() for setting and getting the $name property:

<?php
class Fruit {
// Properties
public $name;
public $color;

// Methods
function set_name($name) {
$this->name = $name;
}
function get_name() {
return $this->name;
}
}
?>

Note: In a class, variables are called properties and functions are called methods!

Define Objects

Classes are nothing without objects! We can create multiple objects from a class. Each object has all the properties and methods defined in the class, but they will have different property values.

Objects of a class are created using the new keyword.

In the example below, $apple and $banana are instances of the class Fruit:

Example

<?php
class Fruit {
// Properties
public $name;
public $color;

// Methods
function set_name($name) {
$this->name = $name;
}
function get_name() {
return $this->name;
}
}

$apple = new Fruit();
$banana = new Fruit();
$apple->set_name('Apple');
$banana->set_name('Banana');

echo $apple->get_name();
echo "<br>";
echo $banana->get_name();
?>

Try it Yourself »

In the example below, we add two more methods to class Fruit, for setting and getting the $color property:

Example

<?php
class Fruit {
// Properties
public $name;
public $color;

// Methods
function set_name($name) {
    $this->name = $name;
}
function get_name() {
    return $this->name;
}
function set_color($color) {
    $this->color = $color;
}
function get_color() {
    return $this->color;
}
}

$apple = new Fruit();
$apple->set_name('Apple');
$apple->set_color('Red');
echo "Name: " . $apple->get_name();
echo "<br>";
echo "Color: " . $apple->get_color();
?>

Try it Yourself »

PHP - The $this Keyword

The $this keyword refers to the current object, and is only available inside methods.

Look at the following example:

Example

<?php
class Fruit {
public $name;
}
$apple = new Fruit();
?>

So, where can we change the value of the $name property? There are two ways:

1. Inside the class (by adding a set_name() method and use $this):

Example

<?php
class Fruit {
public $name;
function set_name($name) {
$this->name = $name;
}
}
$apple = new Fruit();
$apple->set_name("Apple");

echo $apple->name;
?>

Try it Yourself »

2. Outside the class (by directly changing the property value):