PHP has great official documentation already at http://php.net/manual/. The PHP Manual documents pretty much all language features, the core libraries and most available extensions. There are plenty of examples to learn from. The PHP Manual is available in multiple languages and formats.
Best of all, the documentation is free for anyone to edit.
The PHP Documentation Team provides an online editor for the PHP Manual at https://edit.php.net. It supports multiple Single-Sign-On services, including logging in with your Stack Overflow account. You can find an introduction to the editor at https://wiki.php.net/doc/editor.
Changes to the PHP Manual need to be approved by people from the PHP Documentation Team having Doc Karma. Doc Karma is somewhat like reputation, but harder to get. This peer review process makes sure only factually correct information gets into the PHP Manual.
The PHP Manual is written in DocBook, which is an easy to learn markup language for authoring books. It might look a little bit complicated at first sight, but there are templates to get you started. You certainly don't need to be a DocBook expert to contribute.