apt-get install php-pear make libpcre3-dev
pecl install apc
vim /etc/php5/apache2/php.ini
Insert the following line at the top of the file:
extension=apc.so
APC settings are in
/etc/php5/conf.d/apc.ini
Most important is apc.shm_size=xM (x = amount of memory to use)
Note: If you have previously installed APC through apt, remove it first:
apt-get remove php-apc