what is main difference between php4 and php5

PHP5 introduces many new features, I have mentioned some of them:

Unified Constructors and Destructors:
In PHP4, constructors had same name as the class name. In PHP5, you have to name your constructors as  __construct() and  destructors as __destruct().



Abstract:
In PHP5 you can declare a class as Abstract.

Startic Methods and properties:
Static methods and properties are also available. When you declare a class member as static, then you can access members using :: operator without  creating an instance of class.

_autoload()
PHP5 introduces a special function called __autoload()

Final:
PHP5 allows you to declare a class or method as Final 

Magic Methods
PHP5 introduces a number of magic methods.
__call, __get, __set and __toString

Visibility:
In PHP5, There are 3 levels of visibilities:
    Public: Methods are accessible to everyone including objects outside the classes.
    Private: only available to the class itself.
    Protected: accessible to the class itself and inherited class.

Exception:
PHP5 has introduced ‘exceptions’(exception errors)

Passed by reference
In PHP4, everything was passed by value, including objects. Whereas in PHP5, all objects are passed by reference.

Interfaces:
PHP5 introduces interfaces . An interface defines the methods a class must implement. All the methods defined in an interface must be public.

E_STRICT Error Level
PHP5 introduces new error level defined as ‘E_STRICT’
E_STRICT will notify you when you use depreciated code.

PHP5 also introduces new default extensions.

•     SimpleXML: for easy processing of XML data
•     DOM and XSL
•     PDO .
•     Hash :gives you access to a ton of hash functions.

New Functions
PHP5 introduces new functions. You can get a list of them from the PHP Manual.

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 and looks more like ASP.NET now. Here are 10 major differences between PHP4 and PHP5 that you need to know:
1. Unified Constructors and Destructors:
In PHP4, constructors had same name as the class name. This used to cause overhead because every time you changed the class name, you had to change all the occurrences of that name.
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 extend that abstract class. You can include complete method definitions within the abstract methods of abstract class.
3. Final Keyword:
PHP5 allows you to declare a class or method as ‘Final’ now. You just need to use ‘final’ keyword that will indicate that the class cannot be inherited or the method cannot be overridden.
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.
8. Pass 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’
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.
10. New Functions:
PHP5 introduces new functions which are not found in PHP4. You can find the list of these new functions in the PHP manual.