Well all know that this packet is not included into centos distro . To install and use php-mcrypt you have to install and use epel repository.
So here we go
wget http://dl.fedoraproject.org/pub/epel/6/x86_64/epel-release-6-8.noarch.rpm
wget http://rpms.famillecollet.com/enterprise/remi-release-6.rpm
rpm -Uvh remi-release-6*.rpm epel-release-6*.rpm
yum update
yum install php-mcrypt*