Dependency issue trying to install php-mbstring on ec2
It seems you have php 5.6 installed.
You need to install mbstring for that particular version of php.
Run sudo yum install php56-mbstring
After that it might be a good idea to restart apache (thanks! @hexicle),
using sudo service httpd restart
Amazon Linux AMI release 2017.09
sudo yum install php70-mysqlndsudo service httpd restart