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 extension has been deprecated as of PHP 7.1.0 and moved to PECL as of PHP 7.2.0.
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.