CentOS 6.x (64 bit) Installation Notes:
# cd /tmp/
# wget http://pkgs.repoforge.org/libmcrypt/libmcrypt-devel-2.5.7-1.2.el6.rf.x86_64.rpm
# wget http://pkgs.repoforge.org/libmcrypt/libmcrypt-2.5.7-1.2.el6.rf.x86_64.rpm
# rpm -Uhv libmcrypt-*.rpm