PHP APC caching is really easy to install if you are running a repository version of PHP. Make sure you have the development files installed for PHP or it will not be able to phpize APC. Below are step that worked for me on Ubnutu Gutsy with Apache2 for two different machines. (Please note that if you running custom builds of Apache or PHP you will need to compile APC from source as that is the only way I could get it work work.)

1) Open up a terminal window

2) Make sure you have all the files you need. Development PHP files and other necessary make tools using apt-get. Copy and paste the command below.

sudo apt-get install apache2-dev php5-dev php-pear make

2) Install APC with PECL. Copy and paste the command below.

sudo apt-get install apache2-dev php5-dev php-pear make

3) Add the extension to the php.ini file. Open the php.ini file with your favorite editor and search for Modules. Add the line below as a new line.

extension=apc.so

4) Restart Apache (If you are running apache otherwise substitue as necessary).

sudo apache2ctl restart

5) You should notice an immediate jump in speed.

Please note that these steps are almost exactly the same for Fedora Core 4,6, and 7. I have tested it on 6 different servers running Fedora Core and 2 different servers on Ubuntu. You need to use yum instead of apt-get and the package names may vary slight. If you need specifics please let me know and I can help out.