This documentation contains information about current and future PHP versions, 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.
Documentation partly exists for the CVS version of PHP, which always means the very latest development version. This has yet to be released so is only available through the CVS version handling system, or via a snapshot. So be aware that features marked "available in CVS" are typically unavailable. These features, though, will most likely be available in the next stable version of PHP. To download the CVS version, see the anonymous CVS access page.
So, documentation may exist for PHP versions that aren't yet released. (Something like PHP 6.x.x while the latest stable version might be 5.x.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 will be available as 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.