$host = 'stackoverflow.com';
$ports = array(21, 25, 80, 81, 110, 443, 3306);

foreach ($ports as $port)
{
    $connection = @fsockopen($host, $port);

    if (is_resource($connection))
    {
        echo '<h2>' . $host . ':' . $port . ' ' . '(' . getservbyport($port, 'tcp') . ') is open.</h2>' . "n";

        fclose($connection);
    }

    else
    {
        echo '<h2>' . $host . ':' . $port . ' is not responding.</h2>' . "n";
    }
}

stackoverflow.com:21 is not responding.
stackoverflow.com:25 is not responding.
stackoverflow.com:80 (http) is open.
stackoverflow.com:81 is not responding.
stackoverflow.com:110 is not responding.
stackoverflow.com:443 is not responding.
stackoverflow.com:3306 is not responding.