Security Enhancements and Fixes in PHP 5.3.2: The SNMP extension is now available in VC9 versions. For users upgrading from PHP 5.2 there is a migration guide available here, detailing the changes between those releases and PHP 5.3.
For any other SAPI (CLI, Fast CGI via mod_fcgi, Fast CGI with IIS or other Fast CGI capable server), everything works as before.
Third party extension providers must rebuild their extensions to make them compatible and loadable with the Visual Studio C 9 builds that we now provide.
Security Enhancements and Fixes in PHP 5.2.13: Further details about the PHP 5.2.13 release can be found in the release announcement, and the full list of changes are available in the Change Log.
The libc URL library has been updated to 7.20.0, which fixes important bugs fixes as well as a security flaw, see the advisory here and the full changelog here.
For a full list of changes in PHP 5.3.6, see the Change Log.