PHPMD is a spin-off project of PHP Depend and aims to be a PHP equivalent of the well known Java tool PMD. PHPMD can be seen as an user friendly and easy to configure frontend for the raw metrics measured by PHP Depend.
PHPUnit PHPUnit is the de-facto standard for unit testing in PHP projects. It provides both a framework that makes the writing of tests easy as well as the functionality to easily run the tests and analyse their results.
Behat is a tool that makes behavior driven development (BDD) possible. With BDD, you write human-readable stories that describe the behavior of your application. These stories can then be auto-tested against your application.