When you are writing tests for untested legacy code or you do not employ a test-driven development methodology you will frequently find yourself needing to create test cases for already existing classes. PHPUnit has built-in capability to create skeletons for existing classes. This functionality can help you build up your test suite very quickly.
Using this functionality to test legacy code can be very effective. It will create several tests that are marked as incomplete which can be used to help you determine how far away you are from having coverage in all of your class methods.
The skeleton functionality is an add-on that must be installed to PHPUnit. It can be installed via PEAR using the