Michael Dowling has a new post to his site today talking about the latest release for the Guzzle HTTP library and how it now works with RingPHP to make integration life easier. The RingPHP library, inspired by Clojure's Ring library, provides a low-level structure to work with HTTP clients and servers through a simple interface.
With RingPHP, Guzzle does not require cURL and can be used with any HTTP transport mechanism. I'd love to help anyone who is interested in creating RingPHP adapters to bind Guzzle to another library. For example, WyriHaximus on Github is working on binding Guzzle to ReactPHP. (In fact, Guzzle 4 did not require cURL, though it was much harder to use an alternate transport.)
He goes on to talk more about the changes in the Guzzle 5 release including more detail on the RingPHP integration, the use of promises/futures and iterable and callable streams. There's also several new events included in the release as well. He finishes out the post with an upgrade guide to help make the transition easier.