I re-read the post and realized you were talking about integration, heh. I'll extend my comment a bit. So yes, i would write an adapter for Guzzle. If i didn't have to write that integration myself and the case would be using a library like twitteroauth then adapter again with a TwitterApi interface or something.
In case of writing the integration myself i'd be still trying my best to make it irrelevant that i'm using guzzle or even a http client at all. After all, the client probably only cares about the data that the integration provides. This is pretty speculative as i haven't really gotten around to build any kind of meaningful abstraction over these kinds of integrations.
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