PHP OOP – Classes and Objects

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

Object Oriented Concepts

Before we go in detail, lets define important terms related to Object Oriented Programming.

  • Class − This is a programmer-defined data type, which includes local functions as well as local data. You can think of a class as a template for making many instances of the same kind (or class) of object.
  • Object − An individual instance of the data structure defined by a class. You define a class once and then make many objects that belong to it. Objects are also known as instance.

class-object

  • Member Variable − These are the variables defined inside a class. This data will be invisible to the outside of the class and can be accessed via member functions. These variables are called attribute of the object once an object is created.
  • Member function − These are the function defined inside a class and are used to access object data.
  • Inheritance − When a class is defined by inheriting existing function of a parent class then it is called inheritance. Here child class will inherit all or few member functions and variables of a parent class.
  • Parent class − A class that is inherited from by another class. This is also called a base class or super class.
  • Child Class − A class that inherits from another class. This is also called a subclass or derived class.
  • Polymorphism − This is an object oriented concept where same function can be used for different purposes. For example function name will remain same but it take different number of arguments and can do different task.
  • Overloading − a type of polymorphism in which some or all of operators have different implementations depending on the types of their arguments. Similarly functions can also be overloaded with different implementation.
  • Data Abstraction − Any representation of data in which the implementation details are hidden (abstracted).
  • Encapsulation − refers to a concept where we encapsulate all the data and member functions together to form an object.
  • Constructor − refers to a special type of function which will be called automatically whenever there is an object formation from a class.
  • Destructor − refers to a special type of function which will be called automatically whenever an object is deleted or goes out of scope.

Object Oriented Programming 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 goes 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:

Example

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

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

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 is 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();
?>

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();
?>

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");
?>

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

Example

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

PHP – instanceof

You can use the instanceof keyword to check if an object belongs to a specific class:

Example

<?php
$apple = new Fruit();
var_dump($apple instanceof Fruit);
?>
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