get a list of clients with PHP

Hello, there is a problem in a enterprise LAN: On each PC runs a Client, which waits at a haven for messages. With fsockopen(); messages are sent to all existing Clients. Functioned only, if one knows, the which Clients are switched on. If the Script meets a IP, whose PC is switched off, it dies. All following IP's receives no more message. My problem: How can I receive a list of the PC's switched on at present with PHP? System calls are not a genuine solution. The Script is not only to work under Linux. Thank you for your assistance w. b. r. Norbert ______________________________ normally: +49-(0)2292-681769 mobil call: +49-(0)177-2373368 ------------------------------ e.o.m.