static protected function _callFunction($function, &$vars, &$err, $which = null)
	{
		if (!function_exists($function))
		{
			$err = 'Callback does not exist: [function] ' . $function . '(...)';
			return false;
		}

		$ret = call_user_func_array($function, $vars);

		if (!is_null($which))
		{
			$err = $vars[$which];
		}

		return $ret;
	}