public static function string($val, $minLen = 0, $maxLen = null): bool
    {
        if (!\is_string($val)) {
            return false;
        }

        // only type check.
        if ($minLen === 0 && $maxLen === null) {
            return true;
        }

        return self::integer(Helper::strlen($val), $minLen, $maxLen);
    }