yeah do a search on freshmeat.net.
heck it wouldnt even be that hard. i made a script just to monitor an irc log
(link), you could have a little text box which would post text to the end of the file, and have it refresh every 1-3 seconds and show the last N lines, etc.
but i do remember searching at freshmeat and trying out a couple way back when i first started with php (uh, heh, 3 months ago?

), have a look i suppose..
edit: oh another thing, since php is all serverside, theres really no way to have a seamless chat interface with it, you have to refresh every couple seconds.