Everyone knows the www.php.net site. All of us went there sooner or later, and will keep going back there. This is the central reference point for PHP users, and it has a wealth of informations there. All of it isn't that obvious. Come with me, I'll show you.
This is the primary web site. The front page is where major news is published: new PHP versions, security updates, and new projects launched. This site is also mirrored in dozens of countries worldwide.
This is the home of the download page, for everyone to get the latest version of the PHP source code and binaries for Windows. The current and next-to-current versions are available there. (There is also a PHP Museum, which has all of the source distributions since June 1996.)
The next most visited section is the documentation. The documentation is translated into twelve different languages, and is available in a variety of different formats. Users are able to read notes on the documentation left by other users, and contribute their own notes. The documentation is a real community project by itself!
The support page has all the directions to a wealth of resources both inside and outside of PHP.net. The community has built a huge network of knowledge bases, PHP user groups, and training sessions where anyone can have his questions answered. Non-English-speaking users also get a large share of attention.
Now, buckle up your seat belt, and stop smoking. Here are the no-light streets:
This is where speakers at various PHP-related conferencess keep their slides. It covers all sorts of topics, from the famous 'Rasmus' introduction to PHP to the latest 'PHP system administration', through PEAR and advanced topics. All those slides are available within the PHP slide application.
news.php.net is the web interface to the PHP mailing lists. If you're not subscribed to the mailing lists, but you still want to keep in touch regularly, this is your place. An infinite pile of fresh news and trends of PHP. You can also point your news reader at the NNTP server at news.php.net to follow the lists.
PEAR is the next revolution in PHP. This repository is bringing higher level programming to PHP. PEAR is a framework and distribution system for reusable PHP components. It eases installation by bringing an automated wizard, and packing the strength and experience of PHP users into a nicely organised OOP library.
The bug database is where you can bring problems with PHP to the attention of developers (but don't forget to double-check that somebody else hasn't already reported the same problem!).
The Quality Assurance team is one of the most important pieces of the PHP project, protecting users from bugs. It is gathered around the QA mailing list, and this site allows anyone to provide tests and experience to the release process.
The PHP project is organised with a CVS server, and this website is the web interface to it. There you can browse the history (and latest versions) of the source code for all of the PHP projects. For example, the php4 module is the repository for the source code to the latest version of PHP itself.
Bonsai is another window into the CVS repository that makes it easy to search within the sources, so you may keep track of who has done updates to files. This is most handy when you are working in a team on some specific section of the code. You may also keep an eye of evolution of implementation, and know the activity level of every extension.
Cross reference for source code, based on the "Linux Cross Reference". This is the ultimate tool for exploring PHP code. Browse inside the latest PHP code, and anytime important macro and or function are detected within the code, they are linked to their definition, and all their usage locations. This will help you build your code, and understand the PHP source.
This web site is the home of the PHP-GTK project: this allows PHP to be used to build graphical interfaces, with slick interface and highly interactive content. You'll find the downloads and docs here, and the latest news from the project.
Smarty is another PHP spin off: Smarty provides your basic variable substitution and dynamic block functionality, and also takes a step further to be a "smart" template engine, adding features such as configuration files, template functions, variable modifiers, and making all of this functionality as easy as possible to use for both programmers and template designers.
This is your first stop if you're looking for cutting edge development versions of PHP which are generated every day from the current stable and current development sources.