In Downloads and Release notes section on php.net one can find links to download the latest versions of PHP and notes what is new and what was fixed in a new release.
In Documentation section on php.net there are links to PHP manual on different languages. PHP manual consists of all the necessary information about PHP, its installation, language reference, security, features, function reference and apendices. Besides, in the bottom of the page there are links to other PHP related documemtations such as extensions PHP-GTK and PEAR.
In Get Involved section on php.net there is information about becoming a contributing member of PHP community. There are four different ways to contribute: testing, finding/diagnosing failed tests, filing/resolving bug reports and maintain/translate documentation files. The criteria to become a PHP memebr is to be experienced in all four areas plus to know C language. In the end of the Get Involved page there are several useful links for developers(helpful information, discussion, search facilities and mailing list for PHP contributors).
Overall, researching php.net website I found out it is possible to download PHP from it, to find all the helpful information about installation and extensions and even how to become a part of PHP community.