Gimpster - Introductory PHP tutorial geared towards those with little to no PHP experience. Using Strings - Knowing how to properly use strings is extremely important, as is this tutorial. From concatenating strings to magic quotes, this tutorial covers the basics of string use. Edit, delete, add and modify your data. PHP DevCenter - O'Reilly Network's source for PHP developer news and information.
PEAR contains code for common tasks such as database abstraction and error control.
Some PHP conference slide shows, presented at various meetings by PHP professionals. Faqts - A repository of PHP related frequently asked questions. People submit questions and others answer, all duties are very open and public to all. In short, this is an open PHP Knowledge Base. Both online and hard-copy versions are available.