Crypt is a one-way encryption that can't be decrypted. You could use HASH or RIJNDAEL if you want the string to be unencrypted at some point. Both of these allow you to put a random string in a php include file that would be necessary in order to unencrypt so that simply getting access to the database wouldn't be enough and you would need a breech of both the database and php code before any data would be compromised.
The only other option would be to also store the unencrypted version until it is updated the first time.
Did this help? Tips are appreciated...
