I've tested it and it works for me:
set_include_path(__DIR__.'PATH_TO_LIB');
spl_autoload_extensions('.php,.class.php');
spl_autoload_register();
So you've set the correct include_path?
Further for namespaces and classnames there is a standard called PSR-0.
https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-0.md
They also have a SplClassLoader which is a little bigger but also more flexible, although I wonder that you can only set one file extension. Well but search for PSR-0 autoloader
and you'll find more implementations. By the way there are also the standards PSR-1 and PSR-2 which are worth to know..