protected function validate_equalsfield($field, $input, $param = null)
    {
        if (!isset($input[$field]) || empty($input[$field])) {
            return;
        }

        if ($input[$field] == $input[$param]) {
            return;
        }

        return array(
            'field' => $field,
            'value' => $input[$field],
            'rule' => __FUNCTION__,
            'param' => $param,
        );
    }