Anyway to encrypt with a key without using mcrypt?

[Achilles97]

The php server I'm using does not have the mcrypt ability. I want to encrypt a string with a key and be able to decrypt it later (the value will go in a cookie).

Is there anyway to do that? All I've seen are one-way encryptions without a key, which does no good if a user can encrypt a value themselves with that same function.

Thanks!