protected function isString(string $key) : bool
    {
        $conCate = '';
        $columnName = ucfirst($this->convertToFieldName($key)).' should be ';
        if (isset($this->errors[$key.self::ERROR])) {
            list($conCate, $columnName) = $this->setErrorConcat($key);
        }

        $value = $this->param[$key];

        if (!is_string($value) || gettype($value) !== 'string') {
            $this->errors[$key.self::ERROR] = $conCate.$columnName.'valid string';

            return false;
        }

        return true;
    }