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PHP CLASS : Object oriented programming in PHP 1

Object Oriented PHP for Beginners: Steps 1 – 5

For this tutorial, you should understand a few PHP basics: functions, variables, conditionals and loops.

To make things easy, the tutorial is divided into 22 steps.

Step 1:

First thing we need to do is create two PHP pages:

  • index.php
  • class_lib.php

OOP is all about creating modular code, so our object oriented PHP code will be contained in dedicated files that we will then insert into our normal PHP page using php ‘includes’.

In this case, all our OO PHP code will be in the PHP file:

  • class_lib.php

OOP revolves around a construct called a ‘class’. Classes are the cookie-cutters / templates that are used to define objects.

Step 2:

Create a simple PHP class (in class_lib.php)

Instead of having a bunch of functions, variables and code floating around willy-nilly, to design your php scripts or code libraries the OOP way, you’ll need to define/create your own classes.

You define your own class by starting with the keyword ‘class’ followed by the name you want to give your new class.

< ?php 
	class person {
 
	}
?>

Note: You enclose a class using curly braces ( { } ) … just like you do with functions.

Step 3:

Add data to your class

Classes are the blueprints for php objects – more on that later. One of the big differences between functions and classes is that a class contains both data (variables) and functions that form a package called an: ‘object’.

When you create a variable inside a class, it is called a ‘property’.

< ?php 
	class person {
		var $name;
	}
?>

Note: The data/variables inside a class (ex: var $name;) are called ‘properties’.

Step 4:

Add functions/methods to your class

In the same way that variables get a different name when created inside a class (they are called: properties,) functions also referred to (by nerds) by a different name when created inside a class – they are called ‘methods’.

A class’s methods are used to manipulate its own data / properties.

< ?php 
	class person {
		var $name;
		function set_name($new_name) {
			$this->name = $new_name;
		}
		function get_name() {
			return $this->name;
		}
	}
?>

Note: Don’t forget that in a class, variables are called ‘properties’ and functions are called ‘methods’.

Step 5:

Getter and setter functions

We’ve created two interesting functions/methods: get_name() and set_name().

These methods follow a common OOP convention that you see in many languages (including Java and Ruby) – where you create methods to ‘set’ and ‘get’ properties in a class.

Another convention is that getter and setter names should match the property names.

<?php 
	class person {
		var $name; 
		function set_name($new_name) { 
			$this->name = $new_name;  
 		}
 
   		function get_name() {
			return $this->name;
		}
	} 
?>

Note: Notice that the getter and setter names, match the associated property name.

This way, when other PHP programmers want to use your objects, they will know that if you have a method/function called ‘set_name()’, there will be a property/variable called ‘name’.

 

Object Oriented PHP for Beginners: Steps 6 – 11

Step 6:

The ‘$this’ variable

You probably noticed this line of code:

$this->name = $new_name;

The $this is a built-in variable (built into all objects) which points to the current object. Or in other words, $this is a special self-referencing variable. You use $this to access properties and to call other methods of the current class.

function get_name() {
	return $this->name;
}

Note: This may be a bit confusing for some of you … that’s because you are seeing for the first time, one of those built in OO capabilities (built into PHP5 itself) that automatically does stuff for us.

For now, just think of $this as a special OO PHP keyword. When PHP comes across $this, the PHP engine knows what to do.

… Hopefully soon, you will too!

Step 7:

Use your class in your main PHP page. (index.php)

You would never create your PHP classes directly inside your main php pages – that would help defeat the purposes of object oriented PHP in the first place!

Instead, it is always best practice to create separate php pages that only contain your classes. Then you would access your php objects/classes by including them in your main php pages with either a php ‘include’ or ‘require’.

<?php include("class_lib.php"); ?>

Note: Notice how we haven’t done anything with our class yet. We will do that next.

Step 8:

Instantiate/create your object

Classes are the blueprints/templates of php objects. Classes don’t actually become objects until you do something called: instantiation.

When you instantiate a class, you create an instance of it … thus creating the object.

In other words, instantiation is the process of creating an instance of an object in memory. What memory? The server’s memory of course!

< ?php include("class_lib.php"); ?>
< ?php 
	$stefan = new person();
? >

Note:  The variable $stefan becomes a handle/reference to our newly created person object. I call $stefan a ‘handle’, because we will use $stefan to control and use the person object.

If you run the PHP code now, you will not see anything displayed on your pages. The reason for this, is because we have not told PHP to do anything with the object we just created …

Step 9:

The ‘new’ keyword

To create an object out of a class, you need to use the ‘new’ keyword.

When creating/instantiating a class, you can optionally add brackets to the class name, as I did in the example below. To be clear, you can see in the code below how I can create multiple objects from the same class.

… From the PHP’s engine point of view, each object is its own entity.  Does that make sense?

< ?php include("class_lib.php"); ?>
< ?php 
	$stefan = new person();
	$jimmy = new person;
? >

Note:  When creating an object, be sure not to quote the class name.

For example:

$stefan = new 'person';

… will give you an error.

Step 10:

Set an objects properties

Now that we’ve created/instantiated our two separate ‘person’ objects, we can set their properties using the methods (the setters) we created.

Please keep in mind that though both our person objects ($stefan and $jimmy) are based on the same ‘person’ class, as far as php is concerned, they are totally different objects.

< ?php include("class_lib.php"); ?>
< ?php 
	$stefan = new person();
	$jimmy = new person;
	$stefan->set_name("Stefan Mischook");
	$jimmy->set_name("Nick Waddles");
? >

Step 11:

Accessing an object’s data

Now we use the getter methods to access the data held in our objects … this is the same data we inserted into our objects using the setter methods.

When accessing methods and properties of a class, you use the arrow  (->) operator.

<  ?php include("class_lib.php"); ?>
< ?php 
	$stefan = new person();
	$jimmy = new person;
 
	$stefan->set_name("Stefan Mischook");
	$jimmy->set_name("Nick Waddles");
 
	echo "Stefan's full name: " . $stefan->get_name();
	echo "Nick's full name: " . $jimmy->get_name(); 
? >

Note: The arrow operator (->) is not the same operator used with associative arrays: =>.

Congratulations, you’ve made it half way through the tutorial! Time to take a little break and have some tea … OK, maybe some beer.

In a short period of time, you’ve:

  • Designed a PHP class.
  • Generate/created a couple of objects based on your class.
  • Inserted data into your objects.
  • Retrieved data from your objects.

 

 

Object Oriented PHP for Beginners: Steps 12 – 17

Step 12:

Directly accessing properties – don’t do it!

You don’t have to use methods to access objects properties; you can directly get to them using the arrow operator (->) and the name of the variable.

For example: with the property $name (in object $stefan) you could get its value like so:

$name = $stefan->name;

Though doable, it is considered bad practice to do it because it can lead to trouble down the road. You should use getter methods instead – more on that later.

< ?php include("class_lib.php"); ?>		
< ?php 
	$stefan = new person();		
	$jimmy = new person;
 
	$stefan->set_name("Stefan Mischook");
	$jimmy->set_name("Nick Waddles");	
 
	// directly accessing properties in a class is a no-no.
	echo "Stefan's full name: ".$stefan->name;
? >

Step 13:

Constructors

All objects can have a special built-in method called a ‘constructor’. Constructors allow you to initialise your object’s properties (translation: give your properties values,) when you instantiate (create) an object.

Note: If you create a __construct() function (it is your choice,) PHP will automatically call the __construct() method/function when you create an object from your class.

The ‘construct’ method starts with two underscores (__) and the word ‘construct’.

< ?php 		
	class person {
		var $name;
		function __construct($persons_name) {		
			$this->name = $persons_name;		
		}		
 
		function set_name($new_name) {
		 	 $this->name = $new_name;
		}	
 
		function get_name() {		
		 	 return $this->name;		
		 }		
 
	}	 	
? >

For the rest of this tutorial, I’m going to stop reminding you that:

  • Functions = methods
  • Variables = properties

Since this is an OO PHP tutorial, I will now use the OO terminology.

Step 14:

Create an object with a constructor

Now that we’ve created a constructor method, we can provide a value for the $name property when we create our person objects.

You ‘feed’ the constructor method by providing a list of arguments (like you do with a function) after the class name.

For example:

$stefan = new person("Stefan Mischook");

This saves us from having to call the set_name() method reducing the amount of code. Constructors are common and are used often in PHP, Java etc.

< ?php include("class_lib.php"); ?>		
< ?php 
	$stefan = new person("Stefan Mischook");
	echo "Stefan's full name: ".$stefan->get_name();
? >

This is just a tiny example of how the mechanisms built into OO PHP can save you time and reduce the amount of code you need to write. Less code means fewer bugs.

Step 15:

Restricting access to properties using ‘access modifiers’

One of the fundamental principles in OOP is ‘encapsulation’. The idea is that you create cleaner better code, if you restrict access to the data structures (properties) in your objects.

You restrict access to class properties using something called ‘access modifiers’. There are 3 access modifiers:

  1. public
  2. private
  3. protected

‘Public’ is the default modifier.

< ?php 		
	class person {		
	var $name;		
		public $height;		
		protected $social_insurance;
		private $pinn_number;
 
		function __construct($persons_name) {		
			$this->name = $persons_name;		
		}		
 
		function set_name($new_name) {   	
			$this->name = $new_name;
		}	
 
		function get_name() {
			return $this->name;
		}		
 
	}
? >

Note: When you declare a property with the ‘var’ keyword, it is considered ‘public’.

Step 16:

Restricting access to properties: part 2

When you declare a property as ‘private’, only the same class can access the property.

When a property is declared ‘protected’, only the same class and classes derived from that class can access the property – this has to do with inheritance …more on that later.

Properties declared as ‘public’ have no access restrictions, meaning anyone can access them.

To help you understand this (probably) foggy aspect of OOP, try out the following code and watch how PHP reacts. Tip: read the comments in the code for more information:

< ?php include("class_lib.php"); ?>
< ?php  
	$stefan = new person("Stefan Mischook");   
	echo "Stefan's full name: " .  $stefan->get_name() ;  
 
	/*  
	Since $pinn_number was declared private, this line of code 
	will generate an error. Try it out!   
	*/  
 
	echo "Tell me private stuff: ".$stefan->pinn_number;  
? >

Note: If you try to access a private property/variable outside of the class, you will get this:

'Fatal error: Cannot access private property person::$pinn_number in ...'

Step 17:

Restricting access to methods

Like properties, you can control access to methods using one of the three
access modifiers:

  1. public
  2. protected
  3. private

Why do we have access modifiers?

In a nutshell: it comes down to control -it is makes sense to control how people use classes.

The reasons for access modifiers and other OO constructs, can be tricky to understand … especially since we are just beginners here. So give yourself a chance!

That said, we can (summarize and) say that many OOP constructs exist with idea the many programmers may be working together on a project.

< ?php 
	class person {  
		var $name;  
 
		public $height;  
		protected $social_insurance;  
		private $pinn_number;
 
		function __construct($persons_name){   
		   $this->name = $persons_name;  
		}       
 
		private function get_pinn_number(){
			return
			$this->pinn_number;  
		}       
	}   
? >

Notes: Since the method get_pinn_number() is ‘private’, the only place you can use this method is in the same class – typically in another method. If you wanted to call/use this method directly in your PHP pages, you would need to declare it ‘public’.

Object Oriented PHP for Beginners: Steps 18 – 22

Step 18:

Inheritance – reusing code the OOP way

Inheritance is a fundamental capability/construct in OOP where you can use one class, as the base/basis for another class … or many other classes.

Why do it?

Doing this allows you to efficiently reuse the code found in your base class.

Say, you wanted to create a new ’employee’ class … since we can say that ’employee’ is a type/kind of ‘person’, they will share common properties and methods.

… Making some sense?

In this type of situation, inheritance can make your code lighter … because you are reusing the same code in two different classes. But unlike ‘old-school’ PHP:

  1. You only have to type the code out once.
  2. The actual code being reused, can be reused in many (unlimited) classes
    but it is only typed out in one place … conceptually, this is sort-of
    like PHP includes().

Take a look at the sample PHP code:

// 'extends' is the keyword that enables inheritance
class employee extends person 
{
	function __construct($employee_name) {
		$this->set_name($employee_name);
	}
}

Step 19:

Reusing code with inheritance: part 2

Because the class ’employee’ is based on the class ‘person’, ’employee’ automatically has all the public and protected properties and methods of ‘person’.

Nerd note: Nerds would say that ’employee’ is a type of person.

The code:

class employee extends person 
{
	function __construct($employee_name){
		$this->set_name($employee_name);
	}
}

Notice how we are able to use set_name() in ’employee’, even though we did not declare that method in the ’employee’ class. That’s because we already created set_name() in the class ‘person’.

Nerd Note: the ‘person’ class is called (by nerds,) the ‘base’ class or the ‘parent’ class because it’s the class that the ’employee’ is based on. This class hierarchy can become important down the road when your projects become more complex.

Step 20:

Reusing code with inheritance: part 3

As you can see in the code snippet below, we can call get_name on our ’employee’ object, courtesy of ‘person’.

The code:

	<?phpinclude("class_lib.php"); ?>
	<?php
		// Using our PHP objects in our PHP pages. 
		$stefan = new person("Stefan Mischook");
		echo "Stefan's full name: " . $stefan->get_name();
 
		$james = new employee("Johnny Fingers");
		echo "---> " . $james->get_name();
	?>

This is a classic example of how OOP can reduce the number of lines of code (don’t have to write the same methods twice) while still keeping your code modular and much easier to maintain.

Step 21:

Overriding methods

Sometimes (when using inheritance,) you may need to change how a method works from the base class.

For example, let’s say set_name() method in the ’employee’ class, had to do something different than what it does in the ‘person’ class.

You ‘override’ the ‘person’ classes version of set_name(), by declaring the same method in ’employee’.

The code snippet:

<?php
	class person 
	{
		protected function set_name($new_name) {
			if ($new_name != "Jimmy Two Guns") {
				$this->name = strtoupper($new_name);
			}
		}
	} 
 
	class employee extends person 
	{
		protected function set_name($new_name) {
			if ($new_name == "Stefan Sucks") {
				$this->name = $new_name;
			}
		}
	}
?>

Notice how set_name() is different in the ’employee’ class from the version found in the parent class: ‘person’.

Step 22:

Overriding methods: part 2

Sometimes you may need to access your base class’s version of a method you overrode in the derived (sometimes called ‘child’) class.

In our example, we overrode the set_name() method in the ’employee’ class. Now I’ve used this code:

person::set_name($new_name);

… to access the parent class’ (person) version of the set_name() method.

The code:

<?php
	class person 
	{
		// explicitly adding class properties are optional - but 
		// is good practice
		var $name;	 
		function __construct($persons_name) {
			$this->name = $persons_name;
		 }
 
		 public function get_name() {
		 	return $this->name;
		 }
 
		 // protected methods and properties restrict access to 
		 // those elements.
		 protected function set_name($new_name) {
		 	 if ($this->name !=  "Jimmy Two Guns") {
		 	 	$this->name = strtoupper($new_name);
		 	 } 
		}
	} 
 
	// 'extends' is the keyword that enables inheritance
	class employee extends person 
	{
		protected function set_name($new_name) {
		if ($new_name ==  "Stefan Sucks") {
			$this->name = $new_name;
		}
	 	else if ($new_name ==  "Johnny Fingers") {
			person::set_name($new_name);
		} 
	}
 
	function __construct($employee_name) 
	{
		$this->set_name($employee_name);
	}
}
?>

Notes: Using the symbol:

::

… allows you to specifically name the class where you want PHP to search for a method:

'person::set_name()'

… tells PHP to search for set_name() in the ‘person’ class.

There is also a shortcut if you just want refer to current class’s parent – by using the ‘parent’ keyword.

The code:

protected function set_name($new_name) 
{	
	if ($new_name ==  "Stefan Sucks") {
		$this->name = $new_name;	
	 }	
	 else if ($new_name ==  "Johnny Fingers") {
		parent::set_name($new_name);	
	}	
}

 

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