protected function isInt($key) : bool
    {
        $conCate = '';
        $columnName = ucfirst($this->convertToFieldName($key)).' should be ';

        if (isset($this->errors[$key.self::ERROR])) {
            list($conCate, $columnName) = $this->setErrorConcat($key);
        }

        if (filter_var($this->param[$key], FILTER_VALIDATE_INT) === false) {
            $this->errors[$key.self::ERROR] =
                $conCate.$columnName.' integer';

            return false;
        }

        return true;
    }