Real-time updates on the web using PHP?

I am looking to implement real-time collaboration in a document on the web. Is there some library that allows this in PHP? Something similar to SignalR that creates a "persistent connection" using long polling.

update found this library for pubsubhubbub http://code.google.com/p/pubsubhubbub/source/browse/trunk/publisher_clients/php/

Searching by library name led me to How to implement PubSubHubbub?