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!
Solutions:
yum install scl-utils
yum install https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm
yum install http://rpms.remirepo.net/enterprise/remi-release-7.rpm
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
Enjoy!