The Alternative PHP Cache (APC) is a free and open opcode cache for PHP. Its goal is to provide a free, open, and robust framework for caching and optimizing PHP intermediate code.
It can be installed on dedicated servers via WHM. See the Software > Module Installers menu and select PECL
Or via the command line with...
pecl install apc
Or manually by downloading the latest version and compiling it...
# wget http://pecl.php.net/get/APC-3.1.9.tgz
# tar -xvzf APC-3.1.9.tgz
# cd APC-3.1.9
# make install
Then restart apache.
You can update APC using either of the methods above except it's 'pecl update apc' instead of 'pecl install apc'.