Here are a few steps to install Mcrypt in Plesk Panel:
1) Install new Fedora repos (if not already installed)
# 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
2) Install Mcrypt. Ensure that only php-mcyrpt is installed & php version or anything else is not modified
# yum install php-mcrypt
3) After installation, remove epel.repo, epel-testing.repo, remi.repo to avoid unintended changes in future
# cd /etc/yum.repos.d
# mv -iv epel* remi* /etc/yum.repos.d-backup-full/
4) Restart Httpd
# /etc/init.d/httpd restart
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