Robert Basic has a quick post to his site where he shows how to use partial mocks in Mockery, a useful tool for unit testing in PHP applications.
In dealing with legacy code I often come across some class that extends a big base abstract class, and the methods of that class call methods on that big base abstract class that do an awful lot of things. I myself have written such classes and methods in the past. Live and learn.
One of the biggest problems with this kind of code is that it is pretty hard to test. The methods from the base class can return other objects, have side effects, do HTTP calls…
He gives an example of a model that includes a method returning a database connection. In the child class of this the method that he's wanting to test includes a call to this method, making it difficult to test in isolation. He then shows how to make a partial mock of the child class that, be definition, returns a mocked PDO instance instead of the real PDO instance. All other methods will be passed through to the real class.