For those who have debian :
sudo apt-get install php7.0-mbstring
Otherwise in centos (tested in centos 6.6)
sudo yum install php70w-mbstringand as @mpen said you can activate it
sudo phpenmod mbstring sudo apt-get install php7.0-mbstringsudo yum install php70w-mbstringand as @mpen said you can activate it
sudo phpenmod mbstring