It looks like in the extras repo for CentOS 6 the following packages are missing:
libmcrypt.x86_64 : Encryption algorithms library libmcrypt-devel.x86_64 : Development libraries and headers for libmcrypt php-mcrypt.x86_64 : Standard PHP module provides mcrypt library support
These packages exist in the extras repo for CentOS 5.6 and are a critical part of the whole php and mysql interaction.