Wayne May

PHP has a nifty way to automatically load classes without the need to explicitly include class files. There is also a performance benefit, since only the unnecessary class files are loaded.

The traditional way to autoload in PHP was to make use of the __autoload function. The problem with this function is that it can only be used once. So if another library adds an __autoload function, the first one will be overwritten. PHP 5 changed this with then new SPL (Standard PHP Library) autoloader functions.

Autoload functions are now added to a stack and called in order. To add a new function to the stack simply add the callback function with spl_autoload_register ¹. For example:

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function my_autoloader( $classname )
{
  //do something here...
}
spl_autoload_register( 'my_autoloader' );

You can also use a class member to autoload. For example, lets say I wanted to create an Autoloader class, with a load member:

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class Autoloader
{
  public static function load( $classname )
  {
    //do something here...
  } 
}
spl_autoload_register( array( 'Autoloader', 'load' ) );

Callback functions can be unregistered using spl_autoload_unregister ².

If your current code has an __autoload function and you need to support it for legacy reasons, you must explicitly add the __autoload function to the stack.

If your code has an existing __autoload function then it must be explicitly registered on the __autoload stack. Failing to register it would mean that

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if( function_exists( '__autoload' ) ) 
{
    spl_autoload_register( '__autoload' );
}

An example of a very simple autoloader:
index.php

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final class Autoloader
{
  public static function load( $classname )
  {
    $classfile = __DIR__ . "/classes/{$classname}.php";
    if( file_exists( $classfile ) )
    {
      include $classfile;
    }
  } 
}
spl_autoload_register( array( 'Autoloader', 'load' ) );
 
$test = ClassLoadedWithAutoloader();

ClassLoadedWithAutoloader.php

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class ClassLoadedWithAutoloader
{
  //class content
}

In the first file, index.php, I create an object of the class ClassLoadedWithAutoloader, which is in the /classes/ClassLoadedWithAutoloader.php file, but I am never explicitly including it, the autoloader is doing this for me.