when you run sudo apt-get install php5-mcrypt in Ubuntu it doesn't install the extension into the mods-available. You'll have to symlink it.
sudo ln -s /etc/php5/conf.d/mcrypt.ini /etc/php5/mods-available/mcrypt.ini
Then enable the extension and restart Apache.
sudo php5enmod mcrypt
sudo service apache2 reload
I hope this helps you.