I just followed the tutorial located at https://www.digitalocean.com/community/articles/how-to-install-linux-apache-mysql-php-lamp-stack-on-ubuntu while fixing multiple other er
Adding this answer for anyone who encounters this issue with Linux Mint 17.
Just installed a LAMP stack on and was getting this error. The mycrpt.ini file was already inst the /etc/php5/mods-available directory. All that needed to be done was to run the command to enable the module:
sudo php5enmod mcrypt
Restart apache after that and you'll be good to go.