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.
vfsStream is a stream wrapper for a virtual file system that may be helpful in unit tests to mock the real file system.
phploc is a tool for quickly measuring the size of a PHP project.
pdepend can generate a large set of software metrics from a given code base. These values can be used to measure the quality of a software project and they help to identify the parts of an application where a code refactoring should be applied.
phpmd scans PHP source code and looks for potential problems such as possible bugs, dead code, suboptimal code, and overcomplicated expressions
phpDox is the documentation generator for PHP projects. This includes, but is not limited to, API documentation.