I recently faced an script error "Function mb_substr() does not exist". When I checked, there was already mbstring PHP module, installed in my WHM/cPanel based server. But the module didn't work. So why?
Apparently, the mbstring is an additional PHP module that must be activated separately by your own hand. To activate this module, login to your cpanel and find menu "Select PHP Version". Find module called mbstring and check the checkbox, and then click Save.
Now, your script should work.