First error is caused by php because the extension mbstring is either not installed or not active.
The second error is output of phpMyAdmin/your site asking you to install / enable the mysqli extension.
To enable mbstring and mysqli edit your php.ini and add/uncomment the two lines with mbstring.so and mysqli.so on unix or mbstring.dll and mysqli.dll on windows
Unix /etc/(phpX/)php.ini
extension=mysqli.so
extension=mbstring.so
Windows PHP installation folder\etc\php.ini
extension=mysqli.dll
extension=mbstring.dll
Don't forget to restart your webserver after this.
EDIT: User added he was using redhat in the comments so here's how you install extensions on all CentOS/Fedora/RedHat/Yum based linux distros
sudo yum install php-mysqli
sudo yum install php-mbstring
restart your werbserver
sudo /etc/init.d/httpd restart
you can verify your installation with a little php script in your document root. This lists all settings, versions and active extensions you've installed for php
test.php
<?php
phpinfo();