In order to install php-mbstring you have to logon to rhn.redhat.com and modify your default base channel (“Release Channels for Red Hat Enterprise Linux 6 for IA-32 “) to “RHEL Server Optional (v. 6 for 32-bit x86)”. 1.) logon to http://rhn.redhat.com 2.) Systems (tab) 3.) Click on your server link 4.) Click on “Subscribed Channels (Alter Channel Subscriptions)” 5.) Put a checkmar on “RHEL Server