Followings are the source code of Net_Portscan::checkPort.
[[Copy from http://cvs.php.net/co.php/pear/Net_Portscan/Portscan.php?login=2&r=1.3]]
------------------
function checkPort($host, $port, $timeout = 30)
    $socket = @fsockopen($host, $port, $errorNumber, $errorString, $timeout);

    if (!$socket) {
        return NET_PORTSCAN_NO_SERVICE;
    }

    return NET_PORTSCAN_SERVICE_FOUND;
}
------------------

When fsockopen successfuly connect the host/port, it returns resource for socket.  But above code doesn't close socket.
When fsockopen returns resource of socket, the socket should be closed with fclose function.

//Hasegawa