PHP: Hypertext Preprocessor
PHP is a server-side HTML embedded scripting language. It provides web developers with a full suite of tools for building dynamic websites: native APIs to Apache and other web servers; easy access to MySQL, Sybase, Oracle, and other databases; IMAP; LDAP; HTTP headers and cookies. This site is the official home of PHP4.

Zend
A bottom-up rewrite of the PHP scripting engine. It features a modular architecture, vastly improved performance, and greater scalability.

Quality Assurance Team
Voulonteer coders dedicated to providing coders with quality assurance.

PHPMac.com
Dedicated to PHP programmers that use the Mac OS, in particular Apple's OS X unix system. Tutotials, articles and support.

CodingTheWeb: PHP News
PHP development news, tutorials, and technical articles. Available as XML and RSS feeds.

Devshed - Searching with PHP and ht://Dig
Adds search capabilities to site using the popular open source tools PHP3 and ht://Dig.

Making the Case for PHP at Yahoo!
Slides from a talk where Michael J. Radwin makes the case for using PHP over other languages.

ThePHPGuy
PHP, MySQL and Apache articles, tutorials, guides. Also features daily updated developer news as well as a monthly developer newsletter packed with PHP programming tips and tricks.

Wrox Resource Center
PHP support and resource forums run by programmers for programmers, provided by Wrox Press.

PHP Freaks
A community designed to assist developers with PHP and MySQL related information. Includes tips, tutorials, forums, articles, code examples and manuals.

Codewalkers
A community site dedicated to PHP and SQL. Includes user submitted code, tutorials and forums.

International PHP Magazine
Web-based magazine for Web professionals focused on PHP based development. News, online articles, book reviews, forum.

PHPit
PHP and MySQL articles, tutorials and codesnippets. It also has database with commonly asked questions.

PHP Everywhere
John Lim's weblog with comments and reviews. Links to his longer articles and manuals.

PHP Club
A programmer's group united by a desire to develop PHP technologies in the Internet. News, articles, applications and FAQ.