Wiki Pages about PHP (click on topic name above for an automatically-generated list)
Php Language - the main page about the language
PHP Projects
Unit Testing In PHP
Simple Test - similar to Php Unit, but with Mock Object support and Web Tester.
PHP Programming
External links of interest
www.php.net 
- With the best online manual anywhere
www.phpdeveloper.org - PHP Developers Network, news/portal site.
www.phppatterns.com - A pattern site specific to PHP
www.php-help.net - PHP Help:
PHP examples and source codes
www.sitepoint.com - Site Point's advanced PHP forum is the best around.
www.phptherightway.com - PHP 'best practices' guide
phpframeworks.com - There do seem to be a lot of them.
PHP Books
www.php-help.net - PHP Books
Pages with Minor PHP Discussions
PHP Wikis
There are a growing number of Wikis written in PHP, the Php Hypertext Preprocessor scripting language. See the PHP section in Wiki Engines for more.
Here is a list of "active" PHP Wiki Wiki sites as of June 28, 2003:
Php Wiki (1MB+ download with 600 files, GPL, flat files, should be comprehensive but wow, 600 files?).
Wikki Tikki Tavi (29K & 40 files, GPL, many reference links, the latest is version .25 on Source Forge MySQL. Really nice). The instructions are sparse, but there a lot of sites using it.
Phiki Wiki (Small 6-8 program files, flat files, but it wanted to write outside of it's directory -- which I don't allow, but their demo did look nice).
Co Teia (Links off to a site in Brazil (.br) & is in Portuguese).
Puki Wiki (Japanese site).
Wiki Pedia (Probably overkill for most sites... GPL, MySQL, but be warned, it has a lot of hard-coded strings and features specific to the Wikipedia encyclopedia.)
Erfurt Wiki (450K download, very few files, Public Domain, flat files, dbm or mysql/postgres, Funky plugin interface, extensive documentation.)
AnotherPHPWiki (more info here?)
Wakka Wiki (272K 40 files BSD? license, MySQL, very easy install)
Pm Wiki (Nice features, easy install)
Blender Wiki (This one links to a web site in the Netherlands (.nl) won't come up).
Blast Wiki (Links to a page stating Blast Wiki in no more).
Here is a list of Wiki Like Things:
The following PHP Wikis look dead, please update this list if if the come back to life:
Mini Wiki (This one looks dead).
Another Php Wiki (The web site is dead).
Wik Link (a wiki to manage links)
Wiki Type Framework (Maintainer lost time, website is no longer active)
People Using PHP
Adrian Kubala
Marty Schrader (This is the way to do web applications under Linux!)
Marlboro Man (a group of PHP users)
PHP User groups
It's hard to search for pages that are actually about PHP, since many URLs contain ".php" - so this Category should be more useful than most. Or in Google you can try adding intext:php to your query.
Volunteer categorizers are always welcome; start here: www.c2.com
See: Language Gotchas
See original on c2.com