My configuration details
Magento version 2.1
Also installed Mbstring extension,
but still getting error
Call to undefined function Magento\Framework\Event\mb_strtolower()
After throwing reindex command,
If you are using php-fpm you should restart it too, because the new extension is installed but not loaded yet. See similar problem here. You can detect is extension loaded using the
phpinfo();: if the extension was successfully loaded you can see something like this:
To restart php-fpm on ubuntu use the command like:
sudo service php7.0-fpm restart
You can find more info by googling "How to restart php-fpm %your distr%".
For command line, You have setup a php variable.By which you are running php and magento commands on Command line interface. A php.ini is associated with that particular php variable's php configurations. Please check in that file if php_mbstring.dll is enabled or not. Clear chache and remove generation folder in magento-root/var folder.