If none of the above help you out, and you have the option, try obtaining one of the rpm files eg:
wget http://rpms.famillecollet.com/enterprise/6/remi/x86_64/php-mbstring-5.4.45-2.el6.remi.x86_64.rpm
then using rpm, install it ignoring the depenecies like so:
rpm -i --nodeps php-mbstring-5.4.45-2.el6.remi.x86_64.rpm
Hope that helps out.