how to fix - Fatal error: Uncaught Error: Call to undefined function mb_convert_encoding()

I use the php readability library in some of my code, and found on a new server that it was throwing the error.

Fatal error: Uncaught Error: Call to undefined function mb_convert_encoding()

For me this was caused on Debian from php missing the extention:

php-mbstring php7.0-mbstring

which can be installed in debian by using the following command:

sudo apt-get install php-mbstring