If you have installed multiple PHP versions using PPA maintained by Ondrej Surý you may end up with the following error message:
PHP Warning: PHP Startup: mcrypt: Unable to initialize module
Module compiled with module API=20190902
PHP compiled with module API=20170718
This is caused by a misconfiguration: the php.ini file is pointing to the latest version of mcrypt instead of the one compiled for PHP 7.2. To fix that you have to correct the path to the extension. Open the file for editing:
sudo vi /etc/php/7.2/cli/conf.d/mcrypt.ini
and replace extension=/usr/lib/php/20190902/mcrypt.so with extension=/usr/lib/php/20170718/mcrypt.so. Do the same for /etc/php/7.2/fpm/conf.d/mcrypt.ini file.
If you have the module mcrypt.so listed in /etc/php/7.2/cli/php.ini or /etc/php/7.2/fpm/php.ini – delete it to avoid “PHP Warning: Module ‘mcrypt’ already loaded in Unknown on line 0” error