Installing APC php5 cache from pear is a pretty easy task:
apt-get install php-pear
apt-get install php5-dev build-essential
pecl install apc
However, if the last step fails with the following errors,
'apc_regex' has no member named 'preg'
'apc_regex' has no member named 'nreg'
you are missing the perl regular expression libraries on your system!
Installing can be done with
apt-get install libpcre3-dev
as root user.
Executing
pecl install apc
once again will then hopefully work.