Note: If you have plesk panel – check the website for the version of php running. If it is a different version than the standard php installation, create a phpinfo page to check if mcrypt is installed for that php version as a simple “php-m” will just report the standard OS php information.

The first step requires downloading some RPM files that contain the additional YUM repository definitions. The instructions below point to the 64-bit versions that work with our Cloud Server instances.
Centos 5.x

wget http://dl.fedoraproject.org/pub/epel/5/x86_64/epel-release-5-4.noarch.rpm
wget http://rpms.famillecollet.com/enterprise/remi-release-5.rpm
sudo rpm -Uvh remi-release-5*.rpm epel-release-5*.rpm

Centos 6.x

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
sudo rpm -Uvh remi-release-6*.rpm epel-release-6*.rpm

CentOS 7

# sudo yum install epel-release

Or…

The command is as follows to download epel release for CentOS and RHEL 7.x using wget command:

cd /tmp
wget https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm

To install epel-release-7-5.noarch.rpm, type:

# sudo yum install epel-release-latest-7.noarch.rpm

list repos:

# sudo yum repolist

Once installed you should see some additional repo definitions under the /etc/yum.repos.d directory.

$ ls -1 /etc/yum.repos.d/epel* /etc/yum.repos.d/remi.repo
/etc/yum.repos.d/epel.repo
/etc/yum.repos.d/epel-testing.repo
/etc/yum.repos.d/remi.repo

Optional
Enable the remi repository

The remi repository provides a variety of up-to-date packages that are useful or are a requirement for many popular web-based services. That means it generally is not a bad idea to enable the remi repositories by default.

First, open the /etc/yum.repos.d/remi.repo repository file using a text editor of your choice:

# sudo vim /etc/yum.repos.d/remi.repo

Edit the [remi] portion of the file so that the enabled option is set to 1. This will enable the remi repository.

name=Les RPM de remi pour Enterprise Linux $releasever - $basearch
#baseurl=http://rpms.famillecollet.com/enterprise/$releasever/remi/$basearch/
mirrorlist=http://rpms.famillecollet.com/enterprise/$releasever/remi/mirror
enabled=1
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-remi
failovermethod=priority

You will now have a larger array of yum repositories from which to install.

Now run:

# yum install php-mcrypt

# service httpd restart