[Linux] Enable mbstring in linux

use command php -m to check your installed module:

php -m

Install mbstring:

 yum install php-mbstring

if ask Is this ok, type the letter y.
wait for it finish.
Finally, restart your server.

service httpd restart

Reference

http://www.knowledgebase-script.com/kb/article/how-to-enable-mbstring-in-php-46.html