Page 250 - Beginning PHP 5.3

Part II: Learning the Language
Automatically Loading Class Files

Although many of the example scripts in this chapter contain more than one class definition, generally
it ’ s a good idea to keep your classes in separate script files, one class per file. It also helps to name each
class file after the class it contains. For example, you might create a class called Person and store it in a
file called Person.php inside a classes folder (so that you know that Person.php contains a class). Or
if you have created a class called Fruit , you might store it in a file called class.Fruit.php .
Then, when your script needs to create a Person object, it can include the Person.php file to create the
class, then go ahead and create an object from the class:
< ?php
require_once( “classes/Person.php” );
$p = new Person();

? >

require_once() lets you include one PHP script file inside another, which means you can break up
your PHP application into small, manageable script files. You learn more about require_once() and
related functions in Chapter 20 .
Not only does this good practice help to keep your scripts organized and maintainable, but it lets you
take advantage of a nifty feature of PHP: class autoloading .
With autoloading, you create an __autoload() function somewhere in your script. This function should
accept a class name as an argument. Then, whenever another part of your script attempts to create a new
object from a nonexistent class, __autoload() is automatically called, passing in the class name. This
gives your __autoload() function a chance to find and include the class file, thereby allowing the PHP
engine to carry on and create the object.

Here ’ s an example __autoload() function:

function __autoload( $className ) {
$className = str_replace ( “..”, “”, $className );
require_once( “classes/$className.php” );
}

This function stores the name of the nonexistent class in a $className parameter. It then filters this
parameter to ensure it contains no “..” substrings (which could potentially be used by an attacker to
open files or folders above the classes folder). Finally, it calls PHP ’ s require_once() function to load
the file in the classes folder with the same name as the missing class. This should cause the class to be
created, allowing the object in turn to be created from the class.

For example, imagine the same script contained the following code:

$p = new Person;
When the PHP engine encounters the new Person construct, it looks to see if the Person class has been
defined. If not, it calls the previously defined __autoload() function. This in turn includes and runs the
file Person.php inside the classes folder, which creates the class and allows the new Person object to
be created.

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