Installing mcrypt on php7.2

# install dependencies
sudo apt-get -y install gcc make autoconf libc-dev pkg-config
sudo apt-get -y install php7.2-dev
sudo apt-get -y install libmcrypt-dev
# use PECL to install mcrypt

sudo pecl install mcrypt-1.0.1

Since PECL is ancient now, it doesn't actually enable the extension once it's built (unless you have PECL configured so that it knows where your php.ini file is). You'll know that you need to do more if PECL tells you

configuration option "php_ini" is not set to php.ini location
You should add "extension=mcrypt.so" to php.ini

Or, simply ask php:

php  -m

If there's no 'mcrypt' in the output, PHP doesn't know that it's installed.

Although you can add the required line into php.ini; on Ubuntu/Debian you will find module-specific *.ini files in /etc/php/7.2/mods-available/ Just copy one to 'mcrypt.ini', and change the contents so that it contains

extension=mcrypt.so