The SitePoint PHP blog has a recent tutorial showing you how to use the Scrutinizer service to evaluate the quality and "pain points" in your PHP code, be it a library or full application.
We’ve gone through a decent number of tutorials about code quality, inspections, auto-build systems and so on here at SitePoint. [...] In this article, we’ll take a look at Scrutinizer CI – a continuous integration tool that’s quite expensive and closed to private projects, but very handy for public ones.
He starts with a quick comparison of Scrutinizer versus (and really plus) the popular CI service Travis CI. He then walks you through the setup of Scrutinizer to evaluate your application automatically when code is pushed to GitHub. He then gets into the configuration options the service provides including filters, specific checks to evaluate and other tools to execute in the evaluation build. The article then gets into examples of the reports that are provided and a bit of detail about what each view provides. There's also options to hide certain errors that you know aren't actually problems and the "follow up" links it provides for the issues you may not understand.