A solid unit test suite is essential for ongoing development in large projects, especially those with many people involved. Going back and manually testing every individual component of an application after every change is impractical. Your unit tests will help alleviate that by automatically testing your application’s components and alerting you when something is not working the same way it was when you wrote your tests.
The Zend Framework 2 API uses PHPUnit, and so does this tutorial application.
After all the services, DAO, and support classes are tested then it’s time for the controller. Generally, this is hard to test and most developers (based on observation) would rather just test it via Selenium or worse, by hand. That can work but it makes testing logic branches difficult and not to mention it’s time-consuming. Plus no active developer would be willing to wait for browser tests to run before checking in code.