in Ubuntu 14.04

sudo apt-get install php5-mcrypt sudo php5enmod mcrypt 

Ubuntu 16.04

sudo apt-get install php-mcrypt sudo phpenmod mcrypt 

in Ubuntu 14.04

sudo apt-get install php5-mcrypt sudo php5enmod mcrypt 

Ubuntu 16.04

sudo apt-get install php-mcrypt sudo phpenmod mcrypt 

Ubuntu 18.04

sudo apt install php7.0-mcrypt sudo phpenmod mcrypt 

or

sudo apt install php7.2-mcrypt sudo phpenmod mcrypt