kamermans/guzzle4-oauth2-subscriber — pull request #4 by ratza (2014-11-28 13:39:31+00:00)
Title: Add the 5.x releases as possible candidates for this package

It seems that the current release 5.0.3 of Guzzle is compatible with your code. That's not thoroughly tested though. I am using only the Client Credentials grant type. Unfortunately that's all I can test now.

I think it would be nice to be able to use the same library for the newer library version.