do the following:
sudo nano /etc/yum.repos.d/CentOS-Base.repo
under the section updates, comment out the mirrorlist line (put a # in front of the line), then on a new line write:
baseurl=http://centos.intergenia.de/$releasever/updates/$basearch/
now try:
yum install php-mbstring
(afterwards you’ll probably want to uncomment the mirrorlist and comment out the baseurl)