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Here's a quick overview of what has changed between PH4 and PHP5. PHP5 for the most part is backwards compatible with PHP4, but there are a couple key changes that might break your PHP4 script in a PHP5 environment. If you aren't already, I stronly suggest you start developing for PHP5. Many hosts these days offer a PHP5 environment, or a dual PHP4/PHP5 setup so you should be fine on that end. Using all of these new features is worth even a moderate amount of trouble you might go through finding a new host!

Note: Some of the features listed below are only in PHP5.2 and above.

Object ModelThe new OOP features in PHP5 is probably the one thing that everyone knows for sure about. Out of all the new features, these are the ones that are talked about most!

Passed by ReferenceThis is an important change. In PHP4, everything was passed by value, including objects. This has changed in PHP5 -- all objects are now passed by reference.

PHP Code:

$joe = new Person();$joe->sex = 'male';
$betty = $joe;$betty->sex = 'female';

echo $joe->sex; // Will be 'female'

The above code fragment was common in PHP4. If you needed to duplicate an object, you simply copied it by assigning it to another variable. But in PHP5 you must use the new clone keyword.

Note that this also means you can stop using the reference operator (&). It was common practice to pass your objects around using the & operator to get around the annoying pass-by-value functionality in PHP4.

Class Constants and Static Methods/Properties
You can now create class constants that act much the same was as define()'ed constants, but are contained within a class definition and accessed with the :: operator.

Static methods and properties are also available. When you declare a class member as static, then it makes that member accessible (through the :: operator) without an instance. (Note this means within methods, the $this variable is not available)

Visibility
Class methods and properties now have visibility. PHP has 3 levels of visibility:

Public is the most visible, making methods accessible to everyone and properties readable and writable by everyone.
Protected makes members accessible to the class itself and any subclasses as well as any parent classes.
Private makes members only available to the class itself.

Unified Constructors and DestructorsPHP5 introduces a new unified constructor/destructor names. In PHP4, a constructor was simply a method that had the same name as the class itself. This caused some headaches since if you changed the name of the class, you would have to go through and change every occurrence of that name.

In PHP5, all constructors are named __construct(). That is, the word construct prefixed by two underscores. Other then this name change, a constructor works the same way.

Also, the newly added __destruct() (destruct prefixed by two underscores) allows you to write code that will be executed when the object is destroyed.

Abstract ClassesPHP5 lets you declare a class as abstract. An abstract class cannot itself be instantiated, it is purely used to define a model where other classes extend. You must declare a class abstract if it contains any abstract methods. Any methods marked as abstract must be defined within any classes that extend the class. Note that you can also include full method definitions within an abstract class along with any abstract methods.

InterfacesPHP5 introduces interfaces to help you design common APIs. An interface defines the methods a class must implement. Note that all the methods defined in an interface must be public. An interface is not designed as a blueprint for classes, but just a way to standardize a common API.

The one big advantage to using interfaces is that a class can implement any number of them. You can still only extend on parent class, but you can implement an unlimited number of interfaces.

Magic Methods
There are a number of "magic methods" that add an assortment to functionality to your classes. Note that PHP reserves the naming of methods prefixed with a double-underscore. Never name any of your methods with this naming scheme!

Some magic methods to take note of are __call, __get, __set and __toString. These are the ones I find most useful.

Finality
You can now use the final keyword to indicate that a method cannot be overridden by a child. You can also declare an entire class as final which prevents it from having any children at all.

The __autoload FunctionUsing a specially named function, __autoload (there's that double-underscore again!), you can automatically load object files when PHP encounters a class that hasn't been defined yet. Instead of large chunks of include's at the top of your scripts, you can define a simple autoload function to include them automatically.

PHP Code:

function __autoload($class_name) {
     require_once "./includes/classes/$class_name.inc.php";
}

Note you can change the autoload function or even add multiple autoload functions usingspl_autoload_register and related functions.

Standard PHP LibraryPHP now includes a bunch of functionality to solve common problems in the so-named SPL. There's a lot of cool stuff in there, check it out!

For example, we can finally create classes that can be accessed like arrays by implementing the ArrayAccess interface. If we implement the Iterator interface, we can even let our classes work in situations like the foreach construct.

Miscellaneous Features

Type Hinting
PHP5 introduces limited type hinting. This means you can enforce what kind of variables are passed to functions or class methods. The drawback is that (at this time), it will only work for classes or arrays -- so no other scalar types like integers or strings.

To add a type hint to a parameter, you specify the name of the class before the $. Beware that when you specify a class name, the type will be satisfied with all of its subclasses as well.

PHP Code:

function echo_user(User $user) {
    echo $user->getUsername();
}

If the passed parameter is not User (or a subclass of User), then PHP will throw a fatal error.

ExceptionsPHP finally introduces exceptions! An exception is basically an error. By using an exception however, you gain more control the simple trigger_error notices we were stuck with before.

An exception is just an object. When an error occurs, you throw an exception. When an exception is thrown, the rest of the PHP code following will not be executed. When you are about to perform something "risky", surround your code with a try block. If an exception is thrown, then your following catch block is there to intercept the error and handle it accordingly. If there is no catch block, a fatal error occurs.

PHP Code:

try {
    $cache->write();
} catch (AccessDeniedException $e) {
    die('Could not write the cache, access denied.');
} catch (Exception $e) {
   die('An unknown error occurred: ' . $e->getMessage());
}

E_STRICT Error Level
There is a new error level defined as E_STRICT (value 2048). It is not included in E_ALL, if you wish to use this new level you must specify it explicitly. E_STRICT will notify you when you use depreciated code. I suggest you enable this level so you can always stay on top of things.

Foreach Construct and By-Reference Value
The foreach construct now lets you define the 'value' as a reference instead of a copy. Though I would suggest against using this feature, as it can cause some problems if you aren't careful:

PHP Code:

foreach($array as $k => &$v) {
    // Nice and easy, no working with $array[$k] anymore
    $v = htmlentities($v);
}
// But be careful, this will have an unexpected result because
// $v will still be a reference to the last element of the $array arrayforeach($another_array as $k => $v) {

}

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

New Extensions
PHP5 also introduces new default extensions.

SimpleXML for easy processing of XML data
DOM and XSL extensions are available for a much improved XML-consuming experience. A breath of fresh air after using DOMXML for PHP4!
PDO for working with databases. An excellent OO interface for interacting with your database.
Hash gives you access to a ton of hash functions if you need more then the usual md5 or sha1.

Compatibility IssuesThe PHP manual has a list of changes that will affect backwards compatibility. You should definately read through that page, but here is are three issues I have found particularly tiresome:

array_merge() will now give you warnings if any of the parameters are not arrays. In PHP4, you could get away with merging non-arrays with arrays (and the items would just be added if they were say, a string). Of course it was bad practice to do this to being with, but it can cause headaches if you don't know about it.
As discussed above, objects are now passed by references. If you want to copy a object, make sure to use the clone keyword.
get_*() now return names as they were defined. If a class was called MyTestClass, then get_class() will return that -- case sensitive! In PHP4, they were always returned in lowercase.

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