Extreme minification of shortest possible PSR-0 compliant autoloader, 5 lines !

l.php

<?php
//set_include_path(get_include_path() . PATH_SEPARATOR . __DIR__); // optional
spl_autoload_register(function ($class) {
	$file = preg_replace('#\\\|_(?!.+\\\)#','/', $class) . '.php';
	if (stream_resolve_include_path($file))
		require $file;
});

Yeah, i copied the wrong code i think, it's corrected ;)

\namespace\package_name\Class_Name
is resolved as
namespace/package_name/Class/Name.php

About return : I thought returning true could stop the spl autoload stack iteration earlier, but there is no need to do so (spl check if class exists after callink each callback in the stack).

@include (Silent warning) is better than require (Fatal error) because final user could use multiple autolader (with different logic to resolve path) , and we don't know where our autoloader will be in the stack!

@include was lame... Now i check if the file can be included, then require it ;)

Why not use require_once? Or does spl_autoload_register automatically prevent loading files multiple times?

Yes, autoloading happens only if the class definition is not yet included. But may be require_once is safer ;)

This regex is reliable?
Some tests indicate an error in this part "(?!".

For the perfect, please add braces to if statement :))
"The body of each structure MUST be enclosed by braces." (c) PSR-2