In Ubuntu 18.10, mcrypt php module is deprecated.So it isn't available by default.But you could install it using pecl.
To install php-mcrypt module,there are three modules that you need to install in advance.

apt install php-pear php-dev libmcrypt-dev -y
pecl channel-update pecl.php.net

Install the new version of mcrypt that is mentioned in the pecl.php.net
In my case,the version of mcrypt is like

pecl install mcrypt-1.0.2

To Check the mcrypt module

php -m | grep 'mcrypt'

If it results mcrypt, thus mcrypt module is successfully installed.