mCrypt is one of the best and most popular Cryptography Extensions in PHP5. To install it with your PHP5 in your NGinx server for Ubuntu 14.04 please install it with following command
sudo apt-get install php5-mcrypt
and then restart your NGinx server with
sudo service nginx restart. But sometimes it may fails to load your mCrypt extension with your PHP5. If you face those types of problem just execute the following command in your Ubuntu 14.04 command line and restart Nginx.
sudo phpenmod mcrypt
Now you have mcrypt enabled in your PHP5. Keep coding!