The manual contains information about past, current, and future versions of PHP. Changes in behaviour are documented as notes, changelogs, and inline text within the manual pages. The earliest documented version is PHP 5.0.0.
When documentation exists for the latest (unreleased) developmental versions of PHP, it will be labeled as either "available in Git" or "development version." And while these changes should be planned for, in rare cases they may change.
All development takes place in Git and may be checked out as described within the » anonymous Git access page.
And to clarify, the manual will refer to major, minor and point PHP releases. Using PHP 5.3.1 as an example, the 5 refers to the major version, 3 to minor, and 1 is the point release. Typically PHP only adds new features to major and minor releases, and fixes bugs in point releases. However, this convention is not always true.
Also note that the PHP manual is written in present tense, not future tense, even for documented features that are not yet available. The reason for this is so the manual can stand the test of time, thus not require tedious grammar updates with every PHP release.
Many times the PHP manual lists "Default Values" for PHP directives. These values are based on how PHP behaves without a php.ini configuration file, so this may differ from values found in the distributed php.ini-development and php.ini-production files. They also refer to the latest version of PHP, although changelog entries do mention past values. See the PHP directive appendix for details regarding these values and changes.