Samuel Folkes has posted a few of his ideas on the tool(s) that every PHP developer needs to have on his belt to make their work easy and more effective.
Quite often, I am asked the question "What tools do I need for PHP development?". [...] For those who will listen, I have one very simple answer to that question: Aside from a server running PHP and a text editor, you need absolutely nothing. That answer may seem extreme but its the truth. Lets break it down.
Samuel suggests that, while the choices for IDE, debuggers and other tools for PHP development are too many to count, nothing is as good and clean as a basic text editor (his choice is Notepad++). There are a few things that can make your code more effective like phpDocumentor or PHPUnit, but those are more about good coding practices than how you develop.