Fatal error: Call to undefined function mb_strlen()

The function mb_strlen() is not enabled by default in PHP. Please read the manual for installation details:

http://www.php.net/manual/en/mbstring.installation.php

For me the following command did the trick

sudo apt install php-mbstring

On Centos, RedHat, Fedora and other yum-my systems it is much simpler than the PHP manual suggests:

yum install php-mbstring
service httpd restart

To fix this install the php7.0-mbstring package:

sudo apt install php7.0-mbstring

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