This documentation contains information about both PHP 4 and PHP 5, 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. These features, though, 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 while the latest stable version is 4.3.x). 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 which will be available in a known future PHP version. Typically, PHP only adds new features in major releases otherwise only bugs are fixed. Using the A.B.C versioning format, a major release increments A or B whereas minor releases increment C. So for example it's not uncommon for a feature to be documented as available in PHP x.1.x when the latest release is PHP x.0.x. Also note that the manual is written in present tense, not future tense.
Many times the PHP manual lists "Default Values" for PHP directives. These values are based on php.ini-dist and not php.ini-recommended. They also refer to the latest version of PHP. See the PHP directive appendix for details on these values and changes.