Fatal error: Call to undefined function mb_substr()

mb_substr() is a multibyte-safe version of substr(), meaning it works with characters as opposed to bytes. This is most noticeable in UTF-8, where many characters are represented by two or more bytes.

According to the installation instructions, mbstring is not a built-in extension. You must enable it by having the appropriate files and configuring PHP correctly. Some information can be found in the link provided, your webhost should be able to help you with the rest.

To see if mbstring is installed: php -m | grep mbstring

For Linux, install using

sudo apt-get install php-mbstring

Throw this into a terminal:

php -m | grep mb

If mbstring shows up then it should work.