Cannot Initialize mbstring with PHP 7
For those who have debian :
sudo apt-get install php7.0-mbstring
Otherwise in centos (tested in centos 6.6)
sudo yum install php70w-mbstring
and as @mpen said you can activate it
sudo phpenmod mbstring
A solution was reached with the following method:
I put extension=mbstring.so
inside php.ini even though that is no longer the method PHP uses by default to load modules.
From php.ini itself:
Note: packaged extension modules are now loaded via the .ini files found in the directory /etc/php.d; these are loaded by default.
It seemed to fix my problem. I would still be interested to hear if anyone else has a different take on this matter.