I've downloaded Zend framework, but the ZEND_JABBER library isn't included. How can I download it? (http://framework.zend.com/wiki/pages/viewpage.action?pageId=30641)
That page is a proposal for a class for ZF but it doesn't appear to have ever been completed. It never was part of ZF 1 anyway. There don't appear to be any packages for ZF2 that handle Jabber at the moment either. The code for that proposal is available on google code but hasn't been changed since 2007 and may be incomplete.
xmpphp looks like a promising library for xmpp as does JAXL. There are also many other php jabber clients that should all work with your ZF app.