Having problem installing mbstring on your Centos 7 Server running php 5.5.x and php 7.x In my case php-mbstring for php 7.x is missing!
yum install scl-utils
yum install php70-php-mbstring
With this you have PHP7 and php-mbstring extension running next to the other PHP versions!
Don’t forget to restart your apache webserver
systemctl restart httpd