I installed my web application onto a new version of Ubuntu Server, and noted almost immediately that the web application crashed on each login attempt. Consulting the Apache error log, the issue quickly became clear:

Call to undefined function mcrypt_get_iv_size()

It would seem that php_mcrypt.dll is no longer loaded on a default install, meaning that you need to enable the mcrypt module under apache/php. To do this is pretty easy:

sudo apt-get install php5-mcrypt
sudo php5enmod mcrypt
sudo service apache2 restart

Ran through those and everything was now up and running again – so worth noting for anyone else out there busy bashing their heads against a wall! :)