If you are using the php5-mcrypt library in your projects you might experience that it’s no longer working after you have upgraded Ubuntu to 13.10.

You can fix this with the following three commands I have found on stackoverflow [1]:

sudo ln -s /etc/php5/conf.d/mcrypt.ini /etc/php5/mods-available
sudo php5enmod mcrypt
sudo service apache2 restart

[1] http://stackoverflow.com/questions/19446679/mcrypt-not-present-after-ubuntu-upgrade-to-13-10