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
sudo yum-config-manager --enable remi

STEP 2. Install the mcrypt package by typing:

sudo yum -y install mcrypt php-mcrypt