This is how I autoload all the classes in my controllers folder,
# auto load controller classes
    function __autoload($class_name) 
    {
             
        
My version of @Mark Eirich answer:
    function myload($class) {
      $controllerDir = '/controller/';
      $modelDir = '/model/';
      if (strpos($class, 'controller') !== false) {              
        $myclass = $controllerDir . $class . '.php';
      } else {
        $myclass = $modelDir . $class . '.inc.php';
      }
          if (!is_file($myclass)) return false;
          require_once ($myclass);
    }
    spl_autoload_register("myload");
In my case only controller class have the keyword in their name, adapt it for your needs.