I received this error today:
Fatal error: Call to undefined function mb_strlen()
And it would look the same, no matter what multi-byte PHP function I might be using: mb_strpos(), mb_substr(), etc. And it is beginning to be important to have this extensions installed if you are working with international, multi-byte character sets like UTF-8.
The fix is super easy. All you need to do is install the PHP multi-byte extension with yum (if you're on CentOS or RHEL). Do the following from the command line:
yum install php-mbstring
service httpd restart
That installs the extension and restarts Apache. Then you're good to go!