The PHP development group has officially released the latest bugfix release in the PHP 5.3.x series - PHP 5.3.27:

The PHP development team announces the immediate availability of PHP 5.3.27. About 10 bugs were fixed, including a security fix in the XML parser (Bug #65236). Please Note: This will be the last regular release of the PHP 5.3 series. All users of PHP are encouraged to upgrade to PHP 5.4 or PHP 5.5. The PHP 5.3 series will receive only security fixes for the next year.

You can get this latest release from the downloads page (or here for Windows users). As the update fixes a security issue, it's recommended that you upgrade (see this bug).