I’ve implemented a port-knocking client implementation in PHP inspired by the knockd C implementation made by Judd Vinet.
You can can download it at this page.
If you don’t know what port knocking is, it can briefly described as a way of surreptitiously offering services on a server without advertising the fact. By delivering a special sequence of “port knocks”, a user can open up access to a port for his IP only, and close it again when he’s done.
For more information, see the port knocking website or this article that is an introduction to how, using the port-knocking technic, trusted users can manipulate firewall rules to transmit information across closed ports.