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php-mbstring is only available in the rhel optional channel.
This is allowed for EPEL packages, if I'm not wrong. You might create an install code in the Red Hat Network (RHN) and add the optional channel to this install code by default. Any new installation will then avoid this issue for you. Otherwise, please bug Red Hat via a ticket in RHN to move this package to the base channel. CentOS 6 fortunately does not have this issue...