Use call_user_func, as this will work on PHP < 5.4, whereas $callback() does not
Also replace this with call_user_func
Merge pull request #280 from nickygerritsen/master

Use call_user_func, as this will work on PHP < 5.4, whereas $callback() does not

* nickygerritsen/master:
  Also replace this with call_user_func
  Use call_user_func, as this will work on PHP < 5.4, whereas $callback() does not