This one's just a quick link to a howto that's done right - I mean why should I write a post that's already been written way better than I could've done it.
Installing APC on your Apache based webserver makes PHP execute faster by compiling it and caching the result - thus the execution of the same script is pretty darn fast.
Here's the link: http://www.server-world.info/en/note?os=CentOS_6&p=httpd&f=8