Yay, CentOS 6 still doesn’t by default include mcrypt in it’s distribution on repositories.
There is hope, EPEL to the rescue again:
rpm -ivh http://www.mirrorservice.org/sites/dl.fedoraproject.org/pub/epel/6/x86_64/epel-release-6-8.noarch.rpm yum update
You should see EPEL listed as the repos, then:
yum install php-mcrypt
Also a few of you have mentioned you need to restart Apache. To do this:
service httpd restart
Done.
**Please note the above download is for CentOS 6 x86_64**
UPDATE 12/02/2013: Updated to Latest Link for EPEL 6.8