Openfire is a real time collaboration server. It uses the only widely adopted open protocol for instant messaging, XMPP. Openfire is incredibly easy to setup and administer, but offers rock-solid security and performance.
This website provides complete information about the Extensible Messaging and Presence Protocol (XMPP), an open XML technology for presence and real-time communication developed by the Jabber open-source community in 1999.
XMPPHP is the successor to Class.Jabber.PHP that I've been promising for years. Taking advantage of PHP5, I believe it to be an elegant solution with a direct approach.