A quick note for anyone using this function to add their own autoload extensions. I found that if I included a space in between the different extensions (i.e. '.php, .class.php') the function would not work. To get it to work I had to remove the spaces between the extensions (ie. '.php,.class.php'). This was tested in PHP 5.3.3 on Windows and I'm using spl_autoload_register() without adding any custom autoload functions.
Hope that helps somebody.