PHPUnit is a test suite framework that is widely used for PHP projects. It provides improvements over older testing frameworks like SimpleTest, including better command line tools, code coverage reports, assertions, mock objects, and tool chain support. PHPUnit uses annotations and assertions to make tests stricter and more consistent. It supports features like fixtures, mock objects, and code coverage reporting to help test codebases thoroughly. PHPUnit can be run from the command line or a web interface, and integrates with continuous integration servers to run tests automatically.