The PHP Pocket Reference is a handy quick reference for PHP. This small book acts both as a perfect tutorial for learning the basics of PHP syntax and as a reference to PHP. Both online and hard-copy versions are available.
This page contains a collection of "neat" tips and tricks related to PHP. They aren't necessarily related to the coding of PHP but nevertheless, they're pretty useful.
Hidden within this archive are answers to just about every PHP question that comes up. About 150,000 php-general posts live here and it's growing by about 5000 per month. MARC archives many related lists too.