Installing the APC (Alternative PHP Cache) using the php-apc package is straightforward:

# sudo aptitude install php-apc
# sudo /etc/init.d/apache2 restart

Using the package php-apc installs APC 3.1.3p1. If you would like to use the most recent version 3.1.4 you can manually set up the PECL package as follows:

1. Install the required packages:

   # sudo apt-get install php-pear php5-dev apache2-threaded-dev

2. Install APC:

   # sudo pecl install apc-3.1.4

3. Create file /etc/php5/conf.d/apc.ini with the following content:

   extension=apc.so

4. Restart Apache2:

   # sudo /etc/init.d/apache2 restart

After restarting the Apache2 web server a APC section should be included in the phpinfo() output.