Verify php and pecl version

php -v
pecl version

If your php isn’t 7.2 then use /usr/bin/php7.2 instead php command.
Install mcrypt extension

Mcrypt PECL extenstion

sudo apt-get -y install gcc make autoconf libc-dev pkg-config
sudo apt-get -y install libmcrypt-dev
sudo pecl install mcrypt-1.0.1

When you are shown the prompt

libmcrypt prefix? [autodetect] :

Press [Enter] to autodetect.

After success installing mcrypt trought pecl, you should add mcrypt.so extension to php.ini.

The output will look like this:

...
_
Build process completed successfully
Installing '/usr/lib/php/20170718/mcrypt.so' ----> this is our path to mcrypt extension lib
install ok: channel://pecl.php.net/mcrypt-1.0.1
configuration option "phpini" is not set to php.ini location
You should add "extension=mcrypt.so" to php.ini

Grab installing path and add to cli and apache2 php.ini configuration.

sudo bash -c "echo extension=/usr/lib/php/20170718/mcrypt.so > /etc/php/7.2/cli/conf.d/mcrypt.ini"
sudo bash -c "echo extension=/usr/lib/php/20170718/mcrypt.so > /etc/php/7.2/apache2/conf.d/mcrypt.ini"

Verify that the extension was installed

Run command:

php -i | grep "mcrypt"

The output will look like this:

/etc/php/7.2/cli/conf.d/mcrypt.ini
Registered Stream Filters => zlib.*, string.rot13, string.toupper, string.tolower, string.strip_tags, convert.*, consumed, dechunk, convert.iconv.*, mcrypt.*, mdecrypt.*
mcrypt
mcrypt support => enabled
mcrypt_filter support => enabled
mcrypt.algorithms_dir => no value => no value
mcrypt.modes_dir => no value => no value```