If you encounter an error stating: Fatal error: Call to undefined function mb_substr(), this means a particular module is not included in your PHP
configuration. In this article, we will teach you how to resolve the
This error is caused because you are missing the mbstring module within your PHP configuration. While not required by most software, some may use it for multibyte character encoding. If the code that you are using requires multibyte character encoding, you will receive this error when the mb_substr() function is called.
Please note that "mbstring" is a non-default PHP extension. This means it is not enabled by default. - See more at: http://www.knowledgebase-script.com/kb/article/how-to-enable-mbstring-in-php-46.html#sthash.EakvQ5Qj.dpuf
To resolve this error, you will need to install the mbstring PHP module. For more technical assistance, tutorials can be found on the web. I would suggest this link.