mcrypt package is not included in the default CentOS repositories. It can be installed from the Remi repository.
In order to install mcrypt, you need to have root permission.
STEP 1. Enabling the Remi repository
sudo yum install epel-release yum-utils sudo yum install http://rpms.remirepo.net/enterprise/remi-release-7.rpm sudo yum-config-manager --enable remi
STEP 2. Install the mcrypt package by typing:
sudo yum -y install mcrypt php-mcrypt