Dynamic tools

PHPSpec
It allows you to write executable examples reflecting specifications of the desired behaviour of the source code being described.
PHPT
A file-based testing framework for PHP
PHPUnit
Unit testing framework for PHP
Selenium
Testing tool for browser-based testing of web applications. It can be used both for functional, compatability (it has extensive cross-browser support) and regression testing.
SimpleTest
PHP unit tester and mock objects framework
Spike PHPCoverage
Measuring and reporting code coverage for PHP applications. It instruments code, records, and displays line coverage in multiple formats.
XDebug
Stack traces, memory allocation, profiling information for PHP scripts and code coverage analysis
XHProf
Function-level hierarchical profiler for PHP