function process($callback, $mergeFiles = true)
    {
        if (!is_callable($callback)) {
            return self::raiseError(null, QUICKFORM_INVALID_PROCESS, null, E_USER_WARNING, "Callback function does not exist in QuickForm::process()", 'HTML_QuickForm_Error', true);
        }
        $values = ($mergeFiles === true) ? HTML_QuickForm::arrayMerge($this->_submitValues, $this->_submitFiles) : $this->_submitValues;
        return call_user_func($callback, $values);
    }