These web sites and books are excellent companions to Upgrading to PHP 5.
There is a tremendous amount of PHP reference material online. With everything from the annotated PHP manual to sites with periodic articles and tutorials, a fast Internet connection rivals a large bookshelf in PHP documentary usefulness.
The official PHP Manual contains thousands of pages covering all aspects of PHP. It’s an invaluable resource for looking up functions.
Discuss PHP on the PHP mailing lists. Don’t be shy, there’s a list for every topic: programming, databases, and even Windows. A mailing list archive lives at http://news.php.net/.
A great way to keep up-to-date on all the latest PHP developments, this archive contains conference presentation slides.
Don’t reimplement the wheel, download it from PEAR. PEAR—the PHP Extension and Application Repository—contains PHP classes that simplify forms processing, provide a database abstraction layer, generate class documentation, and solve hundreds of other tasks.
PECL is PEAR’s sister. PECL—the PHP Extension Community Library—is a collection of PHP extensions written in C. They’re just like the bundled PHP extensions, except they’re targeted at a specialized audience. PECL contains may useful extensions, including a PHP cache and optimizer, extensions to let you talk to Perl and Python from PHP, and an XML pull parser.
A large collection of PHP articles and tutorials freely available on the web.
A gathering place where members of the PHP community can hang out and meet other PHP programmers.
PHP Cookbook , by David Sklar and Adam Trachtenberg (O’Reilly, 2003).
MySQL Reference Manual , by Michael “Monty” Widenius, David Axmark, and MySQL AB (O’Reilly, 2002); also available at http://dev.mysql.com/doc/.
 While I admit to some bias in favor of PHP Cookbook, I frequently find myself looking up recipes when I need to refresh my memory on a particular topic or technique.