Nikola Posa has a quick post to his site showing how to use a mocked up HTTP request handler - in this case Guzzle - for testing web API clients. These scripts make live HTTP requests to remote APIs as a part of their functionality but this presents a dilemma for testers. Unit tests should not reach out to external sources...and that's where the mock comes in.
Whether you're writing a client for your own web API to offer it to users or you're simply implementing integration for a 3rd-party API in your system, it is important to test it to make sure your client is capable of handling actual API responses correctly.
Testing web API clients is mostly about checking how they deal with responses received after sending requests to API endpoints, and for your unit tests, you introduce test doubles to simulate API calls instead of executing real HTTP requests.
He starts with a quick note about the usual way he's seen this issue worked around, creating a separate mocked class instead of using the actual tool. Fortunately, if you're a Guzzle user, there's a tool that comes with the HTTP client that can be used in your unit tests in much the same way as a normal Guzzle instance: the Guzzle MockHandler. He includes some example code showing how to use this class and inject it into your client and use it in much the same way as a normal client instance.