Vidyut Luther has been looking into testing his application, not with unit tests or performance testing, but with multivariate tests (A/B testing).

I was curious if anyone has any experience with Multivariate (A/B) testing with PHP Frameworks. I found http://phpabtest.com/ yesterday, but I haven’t heard of anyone using it in production yet. Nor have I been able to gather how easily or well it integrates with Symfony, or other frameworks that introduce some level of caching.

He lays out what he's looking to measure, but isn't sure the tool is up to the job. He's open for suggestions as to what the community at large uses for this sort of thing. Besides the one he mentioned, there's a few other PHP-powered ones (some listed here) but nothing that stands out from the crowd.