# yum install php-mbstring
Loaded plugins: downloadonly, fastestmirror, refresh-packagekit, security
Setting up Install Process
Loading mirror speeds from cached hostfile
 * base: mirror.vastspace.net
 * epel: ftp.riken.jp
 * extras: mirror.vastspace.net
 * updates: mirror.vastspace.net
Resolving Dependencies
--> Running transaction check
---> Package php-mbstring.x86_64 0:5.3.3-40.el6_6 will be installed
--> Processing Dependency: php-common(x86-64) = 5.3.3-40.el6_6 for package:php-mbstring-5.3.3-40.el6_6.x86_64
--> Finished Dependency Resolution
Error: Package: php-mbstring-5.3.3-40.el6_6.x86_64 (updates)
           Requires: php-common(x86-64) = 5.3.3-40.el6_6
           Installed: php-common-5.4.35-1.el6.remi.x86_64 (@remi)
               php-common(x86-64) = 5.4.35-1.el6.remi
           Available: php-common-5.3.3-38.el6.x86_64 (base)
               php-common(x86-64) = 5.3.3-38.el6
           Available: php-common-5.3.3-40.el6_6.x86_64 (updates)
               php-common(x86-64) = 5.3.3-40.el6_6
 You could try using --skip-broken to work around the problem
 You could try running: rpm -Va --nofiles --nodigest

yum install php-mbstring --enablerepo=remi

sudo yum list | egrep 'php' | grep 50

ea-php50-php-mbstring.x86_64