PHP.net
The consummate site for all PHP developers – this site is well balanced and is the final word in all things PHP including definitive explanations of all PHP functions complete with code examples.

W3Schools
A good basic resource for trying out simple functions before you test them on your server.  Lacks some more advanced examples but for learning the basics, it’s an excellent resource.

PHP Resource Index
A repository of PHP classes and functions developed under Open Source and available for all developers to use.  If you’re stuck trying to develop a piece of code with a larger purpose, you can probably find it here.