COMET javascript library
There is a jquery comet pluginProbably the best version though is the Dojo library
I have a quick list of commands I used to get a jetty server with comet up and running here
It's not just the javascript library that you need, but a server-side implementation as well. Depending on your needs, you'll need to pick a proper implementation.
You mentioned PHP, which is typically difficult with long-held requests. Other options include having a separate server for your comet requests, or using a SaaS solution, such as WebSync OnDemand.
Depending on which option you pick for the server, you will have one or more javascript clients available. But you need to determine your server implementation first, then use the appropriate javascript implementation to match.