On the PHP::Impact blog, Federico Cargnelutti has posted about test-driven design, specifically with the Symfony framework.
Symfony is one of the few PHP frameworks that gives you basic tools for starting to write tests. [...] Symfony also provides an extension of this class called sfTestBrowser, designed especially for functional tests, which has all the abilities of the sfBrowser object plus some smart assert methods.
He uses this sfTestBrowser object to run an example test on a same get() call's response. It should match the regular expression of "/This is a temporary page/" but doesn't so it fails. Writing up tests like this for new parts of your application first would be considered test-driven development and the Symfony framework makes that simple.