I am woking on another project, and need to be able to create an online instant messenger that combines everybody's chat (Myspace, Facebook, Yahoo, AOL, Google , etc) into one in order to provide the user with an easy way to chat with all their friends and not have to download anything. I was thinking of doing this with PHP, but I'm not exactly sure how I would go about it. I have Googled this, but wasn't able to find any answers.
There are a number of things that you should start looking into here.
Jabber, is an open source system that a lot of Chat clients are built on
JWChat is an open source, web-based system that seems to do roughly what you are looking for. http://blog.jwchat.org/jwchat/-
There is lots of information out there, so starting with a google search for XMPP I think you will find a wealth of information to start you on your project.