Suhosin is an open source patch for PHP

PHP

PHP is a general-purpose server-side scripting language originally designed for web development to produce dynamic web pages. For this purpose, PHP code is embedded into the HTML source document and interpreted by a web server with a PHP processor module, which generates the web page document...

.
"The goal behind Suhosin is to be a safety net that protects servers from insecure PHP coding practices."
In some Linux distribution

Linux distribution

A Linux distribution is a member of the family of Unix-like operating systems built on top of the Linux kernel. Such distributions are operating systems including a large collection of software applications such as word processors, spreadsheets, media players, and database applications...

s (notably Debian

Debian

Debian is a computer operating system composed of software packages released as free and open source software primarily under the GNU General Public License along with other free software licenses. Debian GNU/Linux, which includes the GNU OS tools and Linux kernel, is a popular and influential...

 and Ubuntu

Ubuntu (operating system)

Ubuntu is a computer operating system based on the Debian Linux distribution and distributed as free and open source software. It is named after the Southern African philosophy of Ubuntu...

) it is shipped by default.

Some of the servers using Suhosin include: One Laptop per Child project http://laptop.org, Ubuntu Home Page, http://www.ubuntu.com and http://static.wikipedia.org/