Installing MBstring for php

MBstring

sudo apt-get install php7.1-mbstring
sudo service apache2 restart

If your destination version is different you need to check if the mbstring package exsit or not, an example for php7.0:

sudo apt-cache search php7.0-mbstring

I found it useful to first check existence of all modules that you working with, then performing an upgrade, in addition to that update phpmyadmin after upgrading your php is a good idea