The PHP development team announces the immediate availability of [these new versions]. This is a security release. Many security-related issues were fixed in this release. All PHP 5.5 [and 5.6] users are encouraged to upgrade to this version.
Problems fixed included issues around "user after free" memory bugs, PCRE (regular expression) handling and other smaller issues in other extensions. As mentioned, upgrading is highly recommended to these latest versions, especially given the security fixes involved. You can get these latest releases from the main downloads page (or windows.php.net for the Windows users out there). If you're interested in all of the changes in these releases, check out the full Changelog