There's Suhosin which does a great deal to lockdown the core and has largely limited functions to the essentials http://www.suhosin.org/stories/index.html

On debian, I've used : apt-get install php5-suhosin

My recommendation to use OpenBSD, while a bit of toung-in-cheek, is actually because the PHP (5.4.24 as of the 5.5 OS release) is installed with the patch and Nginx is also chrooted.