In need for a site that explains how to use PHPUnit

What your are looking for is the Pocket Guide. It explaines how to work with PHPUnit from A to Z in several languages. You can read it online or offline, for free, and it's regularly updated.

A great tutorial (git-based, exercise based), on physically going through the motions of testing by Sebastian Marek: https://github.com/proofek/phpnw12-tutorial

Found it very helpful for very baseline beginners. The slides: http://www.slideshare.net/proofek/test-your-code-like-a-pro-phpunit-in-practice