Still can't get it working ..
[updates]
name=CentOS-$releasever - Updates
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&re$
baseurl=http://mirror.centos.org/centos/$releasever/updates/$basearch/
gpgcheck=1
then did the command
root@Web1 [~]# yum install php-mbstring
Loaded plugins: fastestmirror
Setting up Install Process
Loading mirror speeds from cached hostfile
* base: mirrors.clouvider.net
* epel: mirrors.coreix.net
base | 3.7 kB 00:00
extras | 3.4 kB 00:00
updates | 3.4 kB 00:00
No package php-mbstring available.