public function getCombinedPostIndexes(array $fields = null)
    {
        if ($fields === null) {
            $fields = $this->getPOSTFieldNames();
        }

        $keys = array();

        if (count($fields) > 0) {
            if (is_array($this->xdr_post)) {
                foreach ($fields as $field) {
                    if (array_key_exists($field, $this->xdr_post) && is_array($this->xdr_post[$field])) {
                        $keys = array_merge($keys, array_keys($this->xdr_post[$field]));
                    }
                }
            } elseif (is_array($_POST)) {
                foreach ($fields as $field) {
                    if (array_key_exists($field, $_POST) && is_array($_POST[$field])) {
                        $keys = array_merge($keys, array_keys($_POST[$field]));
                    }
                }
            }
            $keys = array_unique($keys);
        }

        return count($keys) > 0 ? $keys : null;
    }