View source for 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

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