In large PHP applications you typically see a “classes” directory that only contains classes which are used throughout the application (database, session management, forms etc..). A problem quickly appears: Everytime you wanted to use one of the classes, you would be forced to include() it at the top of the page. __autoload() solves this by automatically including the definition file when a class is called.

Example

We have a simple application with the index.php file in the root directory and a directory called classes that contains numerous classes.
classes/time.class.php

class time
{
	function fn1()
	{
		echo 'hello world';
	}
}

The code below shows what we’d need to do if we didn’t have autoload. While that code doesn’t seem so bad, imagine writing includes on each of your 30-40 files. It would get messy and very hard to maintain. Another option would be to have a single file that all of your classes are included in, there include that one file in each of your functional scripts but you could expect a performance loss as every one of your classes would be included on the page – even if it wasn’t needed.

index.php

include('classes/time.class.php');

$time = new time();

$time->fn1();

//Returns hello world

The __autoload() function accepts one parameter – the name of the class that is being instantiated. We pass this so we know what file to include.

index.php revised

function __autoload($class_name)
{
	include('classes/' . $class_name . '.class.php');
}

$time = new time();

$time->fn1();

//Returns hello world

A More Complex Example

Large projects and frameworks often have classes split up into groups – extensions, modules and core components. When instantiating a new class, we don’t want to have to declare what group it belongs to. We can build this right into our autoload function, and also check if the file/class exists.

function __autoload($class)
{

	if(file_exists('core/' . $class . '.class.php'))
	{
		include('core/' . $class . '.class.php');
	}

	if(file_exists('extensions/' . $class . '.class.php'))
	{
		include('extensions/' . $class . '.class.php');
	}

	if(file_exists('modules/' . $class . '.class.php'))
	{
		include('modules/' . $class . '.class.php');
	}

}