Call to undefined function mcrypt_module_open

Install mcrypt  based on your php version

sudo apt-get updatesudo apt-get install mcrypt php7.1-mcrypt

or

sudo apt-get-y install gcc make autoconf libc-dev pkg-configsudo apt-get-y install php7.2-devsudo apt-get-y install libmcrypt-devsudo pecl install mcrypt-1.0.1

locate your php.ini file for editing :

php --ini

You should add "extension=mcrypt.so" to php.ini, the location for version php 7.2

sudo vi /etc/php/7.2/cli/php.ini

create a php file and add the following code:

<?php phpinfo();?>

and open it in browser, it will show the php file which is actually being read by apache2. In our case it was

Loaded Configuration File: /etc/php/7.2/apache2/php.ini

You should add "extension=mcrypt.so" to php.ini, the location for version php 7.2

sudo vi /etc/php/7.2/apache2/php.ini

finally we need to restart the apache2 server

service apache2 restart