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
phpdcd is a Dead Code Detector (DCD) for PHP code. It scans a PHP project for code that is no longer used.
phpDox is the documentation generator for PHP projects. This includes, but is not limited to, API documentation.