Smashing magazine has compiled a list of fifty things they think every PHP developer should not be without:

It makes perfect sense to provide the huge community of PHP developers with an overview of useful tools and resources that can make their development process easier and more effective. This post presents 50 useful PHP tools that can significantly improve your programming workflow. Among other things, you'll find a plethora of libraries and classes that aid in debugging, testing, profiling and code-authoring in PHP.

Their long list includes tools is broken up into sections:

• Debugging
• Testing and Optimization
• Documentation
• Security
• Image Manipulation and Graphs
• PHP Code Beautifier
• Version-Control Systems
• Useful Extensions, Utilities and Classes
• PHP Online Tools and Resources
• In-Browser Tools (Firefox Add-Ons)
• Frameworks for PHP
• PHP IDEs and Editors
• Other Sources and Resources