Wow, crazy to see this project still alive and thriving. I was using it 10+ years ago when I worked exclusively in Java. It was great to find those "obvious" issues that slip in that you didn't notice when you should of. I think I had it as a SVN pre-commit hook at one point.

In fact, the lead dev wrote a book "PMD Applied" and I wrote a review of it: http://caseysoftware.com/blog/book-review-pmd-applied

It's since been implemented in PHP too: https://phpmd.org/