Linux setup steps/Install APC as the PHP opcode cache

Installing APC as the opcode cache:

Benchmark before installing, on some PHP page on your site:

ab -kc 10 -t 30 http://yourhostname.com/some_php_page.php

Install required dependencies:

aptitude install apache2-dev
aptitude install build-essential

Install APC (choose "yes" when prompted) :

pecl install apc

Then:

nano /etc/php5/apache2/php.ini

... and add these 3 lines to the end:

; APC opcode cache:
extension=apc.so
apc.mmap_file_mask=/tmp/apc.XXXXXX

Then reload apache:

/etc/init.d/apache2 restart

Check that there are no errors in the error log:

tail -20 /var/log/apache2/error.log

Benchmark after installing:

ab -kc 10 -t 30 http://yourhostname.com/some_php_page.php

For me this showed an increase from 895 completed requests to 3728 completed requests on a test box.
For me this showed an increase from 2889 completed requests to 12023 completed requests on a production box.

That's around a 4.1 times speed increase on two systems - not bad.