The PHP.net website has introduced a new feature to help make it a bit clearer which versions of PHP are supported and which have reached their end-of-life mark. This new Supported versions page off the main site provides listings of currently supported versions and graphical timelines of past (and future) support milestones.
Each release branch of PHP is fully supported for two years from its initial stable release. During this period, bugs and security issues that have been reported are fixed and are released in regular point releases. After this two year period of active support, each branch is then supported for an additional year for critical security issues only. Releases during this period are made on an as-needed basis: there may be multiple point releases, or none, depending on the number of reports.
The page includes information on when the initial release in a series was made (like the 5.4.x or 5.5.x series), when active support did/will end and how long the timeline is for security fixes and support. As of the time of this post, PHP 5.3.x is the only series that has reached end-of-life, but the 5.4.x series is coming close being in security fix only mode now and EOL-ing completely in ten months.