public static function getVersion(array $server = null)
    {
        $protocol = static::make($server);

        if (! preg_match('#\A(?:HTTP/)?(?P<version>\d{1}\.\d+)\Z#', $protocol, $matches)) {
            throw new UnexpectedValueException(sprintf(
                'Unrecognized protocol version "%s".',
                $server['SERVER_PROTOCOL']
            ));
        }

        return $matches['version'];
    }