The PHP project has officially released the latest versions in both the 5.3.x and 5.4.x series in response to a bug that was found in the CGI setup of certain server+PHP configurations.
There is a vulnerability in certain CGI-based setups (Apache+mod_php and nginx+php-fpm are not affected) that has gone unnoticed for at least 8 years. Section 7 of the CGI spec states: 'Some systems support a method for supplying a [sic] array of strings to the CGI script. This is only used in the case of an `indexed' query. This is identified by a "GET" or "HEAD" HTTP request with a URL search string not containing any unencoded "=" characters.'
A large number of sites run PHP as either an Apache module through mod_php or using php-fpm under nginx. Neither of these setups are vulnerable to this. Straight shebang-style CGI also does not appear to be vulnerable. If you are using Apache mod_cgi to run PHP you may be vulnerable. To see if you are, just add ?-s to the end of any of your URLs. If you see your source code, you are vulnerable. If your site renders normally, you are not.
You can download this latest version from the downloads page for the source releases or windows.php.net for the Windows binaries. You can look at the Changelog if you'd like more details on the update.