You will need the mcrypt extension, with also requires libmcrypt, libmcrypt-devel.
PHP must then be compiled with the --with-mcrypt=/path/to/mcrypt
I'm assuming wamp has this already compiled in, you should be able to enable the extension somewhere.

I've got no knowledge of CI so I can't help you there I'm afraid.

http://www.php.net/manual/en/book.mcrypt.php

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mysql> SELECT * FROM `users` WHERE `users`.`clue` > 0;
Empty set (0.00 sec)