CentOS 6.x doesn’t by default include mcrypt in it’s distribution on repositories. Problem can be solved by installing EPEL repository.

rpm -ivh http://download.fedoraproject.org/pub/epel/6/x86_64/epel-release-6-5.noarch.rpm
yum update
yum install php-mcrypt

**Please note the above download is for CentOS 6 x86_64**

Enjoy!