In a new post over on DZone.com Giorgio Sironi shows how to create a fake, an interface that mocks the interface to an external tool and provides cached results back to the calling script.
We can specialize our Fake in a Self-Initializing Fake, which will provide an alternative implementation with the aid of the real one. In our Google Maps case, the Fake will use the real web service for its first response, and maintain an internal cache. This mechanism provides insurance about out-of-sync responses, and lets you enjoy the speed of unit tests after the initial warmup: if you always use the same data, no duplicate requests will be made to the external resource.
He includes example code for creating a test (with PHPUnit) that loads in the information from the Google Maps web service and caches it into a private variable inside the Fake. His test fails the first time checking for a difference in the time between fetching the real version and the cached version (fails the first time, but passes after the cache is implemented).