This extension rely in libmcrypt which is dead, unmaintained since 2007.
Please don't rely on it, consider switching to well maintained alternatives (openssl, crypt, password hashing functions, phpseclib, password_compat...)
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.
This extension rely in libmcrypt which is dead, unmaintained since 2007.
Please don't rely on it, consider switching to well maintained alternatives (openssl, crypt, password hashing functions, phpseclib, password_compat...)
if you get a "The mcrypt extension is missing" alert somewhere, use
"sudo php5enmod mcrypt" and restart your server to enable it.
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