Differences between PHP4 and PHP5

PHP5 is a lot different than PHP4. With the vastly improved Object Oriented model in PHP5, PHP is now a lot closer to a fully fledged object orientated programming language .

Following differences between PHP4 and PHP5 .

 1. Unified Constructors and Destructors:

In PHP 4 the constructor was just a method with the same name(Class) as the name of the class. So, if you changed the name of the class, you had to go and update it every time it was used. 

In PHP 5, to spare the coders this hassle, the PHP developers have created an unified name for the constructors - "__construct()".

A new addition word is the "__destruct()" keyword. When used, the code will be executed only when the object is destroyed.

In PHP5, you simply need to name your constructors as __construct(). (the word ‘construct’ prefixed by double underscores). Similarly you can name your destructors as __destruct(). (the word ‘destruct’ prefixed by double underscores.) In destructors, you can write code that will get executed when the object is destroyed.

2. Abstract Class:

PHP5 lets you declare a class as ‘Abstract’. (i.e. a class whose object cannot be created. You can only extend an abstract class) Also, a class must be defined as abstract if it contains any abstract methods. And those abstract methods must be defined within the class which extends that abstract class. You can include complete method definitions within the abstract methods of abstract class.

Here is how a normal class is defined:

class Message{

and here is how an abstract class is defined:

abstract class Message{

3. Final Keyword:

PHP5 allows you to declare a class or method as ‘Final’ now. The "final" keyword has been introduced, so that a method cannot be overridden by a child now. This keyword can also be used to finalize a class in order to prevent it from having children.

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4. Exception Handling:

PHP5 has introduced ‘exceptions’. An exception is simply a kind of error and the ‘exception error’ can be handled in an exception object. By using an exception, one can gain more control over the simple trigger_error notices we were stuck with before.

When you are about to perform something ‘risky’ in your code, you can surround your code with a ‘try…catch’ block. First you surround your code in a ‘try {…….}’ block, then if an exception is thrown, your following ‘catch{……}’ block is there to intercept the error and handle it accordingly. You can write some PHP code in your ‘catch’ block which will get executed when an error occurs in the ‘try’ block. If there is no ‘catch’ block, a fatal error occurs.

5. E_STRICT Error Level:

PHP5 introduces new error level defined as ‘E_STRICT’ (value 2048). This error levels notifies you when you use depreciated PHP code. It is not included in E_ALL, if you wish to use this new level you must specify it explicitly.

6. Autoloading (the __autoload() function):

PHP5 introduces a special function called ‘__autoload()’ (the word ‘autoload’ prefixed by double underscores). This function allows you to avoid writing a long list of includes at the top of your script by defining them inside this function. So you can automatically load object files when PHP encounters a class that hasn’t been defined yet.

Example:

function __autoload ($class_name) {

include $class_name . '.php';

}

7. Visibility:

In PHP5, class methods and properties now have ‘visibility’. There are 3 levels of visibilities:

Public: ‘Public’ is the most visible. Methods are accessible to everyone including objects outside the classes. And properties readable and writable by everyone including objects outside the classes.
Private: ‘Private’ makes class members only available to the class itself.
Protected: ‘Protected’ makes class members accessible to the class itself and any inherited class (subclass) as well as any parent classes.

PHP4′s method of declaring a variable as ‘var’ keyword is still supported in PHP5. The ‘var’ keyword is now a synonym for the ‘public’ keyword now.

Here is an example of how members are declared

<?php
/**
  * Define ClassA
  */
class ClassA
{
     public $public = 'Public';
     protected $protected = 'Protected';
     private $private = 'Private';

    function printHello()
     {
         echo $this->public;
         echo $this->protected;
         echo $this->private;
     }
}

$bor = new ClassA();
echo $bor->public; // Will work
echo $bor->protected; // Will give you a fatal error
echo $bor->private; // Will give you a fatal error
$obj->printHello(); // Will display Public, Protected and Private

/**
  * Define ClassB
  */
class ClassB extends ClassA
{
     // we can redeclare both the public and protected method, but we can't redeclare the private one
     protected $protected = 'Protected2';

    function printHello()
     {
         echo $this->public;
         echo $this->protected;
         echo $this->private;
     }
}

$obj2 = new MyClass2();
echo $obj2->public; // Will work
echo $obj2->private; // Is now undefined
echo $obj2->protected; // Will display a fatal error
$obj2->printHello(); // Will show you Public, Protected2, Undefined

?>

8. Pass by Reference:

In PHP 4 everything, including objects, was passed by value. This has been changed in PHP 5 where everything is passed by reference.

In PHP4, everything was passed by value, including objects. Whereas in PHP5, all objects are passed by reference. Take a look at this PHP4 code for example -

$peter = new Person();
$peter->sex = ’male’;

$maria = $peter;
$maria->sex = ’female’;

echo $peter->sex; // This will output ‘female’

As you can see in the code above, if you wanted to duplicate an object in PHP4, you simply copied it by assigning it to another variable (Pass by value). But now in PHP5 you must use the new ‘clone’ keyword. So the above PHP4 code, will now look like this in PHP5 -

$peter = new Person();
$maria = new Person();

$peter->sex = ’male’;

$maria = clone $peter;
$maria->sex = ’female’;

echo $peter->sex; // This will output ‘female’

PHP Code:

$pObject1 = new Object();
$pObject1->setName('Adam');
$pObject1->setAddress('http://www.google.com/');

$pObject2 = new Object();
$pObject2->setName('Karl');
$pObject2->setAddress('http://www.google.com/');

This is a typical PHP 4 code - if you wanted to duplicate an object, you had to copy it and assign a new value to it. In PHP 5 the coder can simply use the “clone”. This also means that you no longer need to use the reference operator (&) for your code.

Here is how the same code will look in PHP 5 :

$pObject1 = new Object();
$pObject1->setName('Adam');
$pObject1->setAddress('http://www.talkphp.com/');
$pObject2 = clone $pObject1;
$pObject2->setName('Karl'); 

Since we were chaning only the name, we "cloned" the first object and simply changed the value that needed changing.

9. Interfaces:

PHP5 introduces ‘interfaces’. An interface defines the methods a class must implement. All the methods defined in an interface must be public. An interface helps you design common APIs. It is not designed as a blueprint for classes, but just a way to standardize a common API. A big advantage of using interfaces is that a class can implement any number of interfaces. You can still only ‘extend’ on parent class, but you can ‘implement’ an unlimited number of interfaces.

A big advantage of this new addition is that in a class you can implement any number of interfaces.

Here is how it all works :

an example of a simple class:

class cow {
function moo() {
echo "moo, moo, moo …";
}
}

and now we implement the interface in the class:

class cow implements animal{ 
function moo() {
echo "moo, moo, moo …";
}
function breath() { echo "cow is breathing …";}
function eat() { echo "cow is easting …";}
}

When an interface is implemented in a class, the class MUST define all methods and functions of the interface, otherwise the php parser will show a fatal error.

10. Class Constants and Static Methods/Properties
With PHP 5 you can safely create class constants that act in very much the same way as do defined constants, but are limited within a class definition and can be accessed with “::”. Have in mind that constants must have a constant expression for a value; they can't be equal to a variable or a result of a function call.

Here is how you can define a constant:

PHP Code:

const constant = 'constant value';

And here is how the constant can be accessed in a defined class:

PHP Code:

class MyClass
{
   const constant = 'constant value';

   function showConstant() {
       echo  self::constant . "\n";
   }
}

The Static Methods and Properties are also a PHP 5 innovation. When a class member is declared as static, it's accessible with "::" without an instance. 

11. Magic Methods
All methods, starting with a double underscore ("__") are defined as "Magic Methods". They are set to additional functionality to the classes. It's recommended that you don't use methods with the same naming pattern.

Some of the most used magic methods are: __call, __get, __set and __toString.