If you use an anonymous function, it will return the object that are expected.
spl_autoload_register(function ($myclass){
    $keyclass = substr($myclass, 0, 1);
    switch ($keyclass) {
        case 'c':
            if (file_exists("class".DIRECTORY_SEPARATOR.$myclass.".php") === true)
                require_once ("class".DIRECTORY_SEPARATOR.$myclass.".php");
            break;
        case 'i':
            if (file_exists("interface".DIRECTORY_SEPARATOR.$myclass.".php") === true)
                require_once ("interface".DIRECTORY_SEPARATOR.$myclass.".php");
            break;
        case 'a':
            if (file_exists("abstract".DIRECTORY_SEPARATOR.$myclass.".php") === true)
                require_once ("abstract".DIRECTORY_SEPARATOR.$myclass.".php");
            break;
        default:
            if (file_exists($myclass.".php") === true)
                require_once ($myclass.".php");
    }
/******************************/
var_dump(spl_autoload_functions()) return:
array(1) {
  [0]=>
  object(Closure)#1 (1) {
    ["parameter"]=>
    array(1) {
      ["$myclass"]=>
      string(10) "<required>"
    }
  }
}