PhpMyAdmin not working after PHP 7 update in Ubuntu
How To Install and Secure phpMyAdmin on Ubuntu 14.04 : https://www.digitalocean.com/community/tutorials/how-to-install-and-secure-phpmyadmin-on-ubuntu-14-04
The mbstring extension is missing. – Solution:
sudo apt-get install libapache2-mod-php7.0
sudo apt-get install php7.0-mbstring
and then sudo service apache2 restart.