This documentation contains information about PHP 4, with some added migration and compatibility notes regarding PHP 3. Behaviour, parameter, return value and other changes between different PHP versions are documented in notes and inline text in the manual.
You may find documentation pieces for the CVS version of PHP, which always means the very latest development version available through the CVS version handling system. If you are not a developer of PHP itself, and you are not keen on using the very latest development version of PHP, features marked with the "available in CVS" wording are not accessible to you. Though these features will probably be available in the next stable version of PHP. If you would like to download the CVS version, see the anonymous CVS access page.
You may also encounter documentation for a PHP version which is not released (something like PHP 5.0.0 as the latest stable version is 4.3.0). Most of the time, this is not an error in the documentation. Explanation is often added for features not available in the current PHP release, but will be available in a known future PHP version.