GUMP::add_validator PHP Method

    public static function add_validator($rule, $callback)
    {
        $method = 'validate_' . $rule;
        if (method_exists(__CLASS__, $method) || isset(self::$validation_methods[$rule])) {
            throw new Exception("Validator rule '{$rule}' already exists.");
        }
        self::$validation_methods[$rule] = $callback;
        return true;
    }

 public function save()
 {
     GUMP::add_validator("unique", function ($field, $input, $param = NULL) {
         $checkExistingUser = R::findOne('user', 'user=?', array($input));
         if ($checkExistingUser == NULL) {
             return FALSE;
         } else {
             return TRUE;
         }
     });
     GUMP::add_validator("strong", function ($field, $input, $param = NULL) {
         return checkPasswordStrength($input);
     });
     $rules = array('reseller_username' => 'required|alpha_numeric|max_len,10|min_len,6|unique', 'reseller_password' => 'required|max_len,10|min_len,7|strong');
     $filters = array('reseller_username' => 'trim|sanitize_string', 'reseller_password' => 'trim|sanitize_string|md5');
     $app = Slim::getInstance();
     $post = $app->request()->post();
     // $app - Slim main app instance
     $postValues = $gump->filter($post, $filters);
     $validated = $gump->validate($gump->filter($postValues, $filters), $rules);
     if ($validated === TRUE) {
         $createUser = R::dispense('user');
         $createUser->user = $postValues['reseller_username'];
         $createUser->user = $postValues['reseller_password'];
     } else {
         $this->setError($gump->get_readable_errors(true));
     }
     if ($this->getError() == "") {
         $this->fails = FALSE;
     } else {
         $this->fails = TRUE;
     }
 }