In this recent post to his blog Johannes Schluter talks about a method he's suggested for testing objects in unit tests - overriding the "new" operator to replace specific classes with mocks.

Two years ago at some conference I had a conversation with Sebastian about the need for a way to overload the new operator in PHP so, probably, bad designed code can be tested more easily by replacing specific classes with mocks. [...] Sebastian then pushed the code as part of a new test_helpers extension with some documentation to github and I fixed some bugs in it. The aim of the extension is to collect functionality which might be beneficial for phpUnit and other test scenarios but which should never reach a production environment.

He includes some sample code to show it in action - defining the mock class, using the set_new_overload function to define it as what should be called when the "new" operator is used and a dump of the result.