Security Enhancements and Fixes in PHP 5.3.9:
Added max_input_vars directive to prevent attacks based on hash collisions. (CVE-2011-4885)
Fixed bug #60150 (Integer overflow during the parsing of invalid exif header). (CVE-2011-4566)
Key enhancements in PHP 5.3.9 include:
Fixed bug #55475 (is_a() triggers autoloader, new optional 3rd argument to is_a and is_subclass_of).
Fixed bug #55609 (mysqlnd cannot be built shared)
Many changes to the FPM SAPI module