In linux systems,mcrypt is necessary for installing CMSes like majento. First You need to install the mcrypt library.
If using a Debian-based Linux system, you can run the following commands:
sudo apt-get install php5-mcrypt
then restart the apache server
sudo /etc/init.d/apache2 restart
This is an interface to the mcrypt library, which supports a wide variety of block algorithms such as DES, TripleDES, Blowfish (default), 3-WAY, SAFER-SK64, SAFER-SK128, TWOFISH, TEA, RC2 and GOST in CBC, OFB, CFB and ECB cipher modes. Additionally, it supports RC6 and IDEA which are considered "non-free". CFB/OFB are 8bit by default.