PHP - __autoload()

I'm trying to use the autoload class but am having some trouble.  Here is my autoload class,

Code: [Select]
function __autoload($class_name) {
    include("classes/" . $class_name . ".class.php");
}

The problem I am having is that when I include this class at the top of this page I then keep getting this error - "Warning: Cannot modify header information - headers already sent by" whenever I try to start a session or use the "header" function but I'm not sure why that's happening because surely I haven't echoed anything yet. 

Thanks for any help.