If you already have a set of unit tests (PHPUnit) and are thinking of implementing a build tool like Phing, you might want to check out this new post from Giorgio Sironi about combining the two with a simple addition to your build configuration.
Integrating these two tools means giving Phing access to a PHPUnit test suite and letting the Phing build files, which manage configuration, contain also information on how to run the test suite. In the build.xml file of an application you should find different targets like generate-documentation, test-all, compile-all (if php were a compiled language), and so on.
He mentions the two ways you can run your unit tests - one being more native (a phpunit task) and the other a bit more "blind" (exec task). He looks at the first, allowing tighter integration with Phing and can actually break the build if something fails. He includes the XML for both to include in the build file and a brief explanation of what they're doing behind the scenes.