Davey Shafik has published a post about library he's created that's a sort of "recorder" for connections made with the Guzzle HTTP client - the Guzzle VCR.

A few days ago I pushed out a very small library to help with testing APIs using Guzzle: dshafik/guzzlehttp-vcr. [...] This is a simple middleware that records a request’s response the first time it’s made in a test, and then replays it in response to requests in subsequent runs.

The handler works by recording the responses from the API (ex: the JSON response data) and records them to files (again, JSON). A one-line call turns the "recording" on and points to a directory where the cached files should be stored. He shows how to use it in the constructor of your Guzzle client, setting it up as the "handler" for the requests. He also includes an example of a few unit tests that make use of the recording feature to check the response of a /test endpoint.