According to I-nfotech, there's ten projects that every PHP developer out there just can't live without:

As a php web developer, you should know that php is probably the language that has the biggest code repository. So no matter what module you want to include in your project there should be an open source solution. [...] Anyway, after many years as a web developer, I've compiled a list of php classes that can be easily integrated in any project and I am regularly use.

He has it broken up into the ten different categories including "sending emails", "user manipulation", "fetching RSS feeds" and "grabbing remote content". Some of his choices are things native to PHP (like the mail function) but others are external classes like the Magpie RSS package.