PHP is not designed to run a game server. Doing one in PHP would be really stupid. If the only reason is to have a cross-platform source or because you don't know other languages, well, that's pretty bad reasons...
COPS v7 is the only source on this forum that I'm aware of to be designed with cross-platform support in mind. It will work on any UNIX-like system (Unix, GNU/Linux, OS X) and on Windows systems. Also, it should work on any processor architecture (tested at least with Intel (i386, x86_64), PowerPC (ppc, ppc64) and ARM). Although I don't recommend it, you could run it on a Raspberry Pi...
COPS v6 could work with Mono with some minor modifications (I have a working version).