MCrypt is a replacement for the old crypt package and crypt command with 
extensions. It allows developers to use a wide range of encryption functions 
including encrypting files or data streams without having to be cryptographers.

The companion to MCrypt is libmcrypt, which contains the actual encryption 
functions themselves and provides a standardized mechanism for accessing them.

mcrypt and libmcrypt packages are recommended if you are using phpmyadmin for 
database administration.

To actually use mcrypt, you would have to recompile php with the following 
config switch:  "--with-mcrypt=shared,/usr/bin"