What exactly are you trying to achieve? If you use HTTPS on your site then the "transmission" of the messages (sending them to the server and reading them from the server) will be encrypted.
They won't be encrypted on disk, but I'm not sure that's necessary. I don't know of any other companies or apps doing that. If users are worried about administrators reading their PMs in the database, well there isn't any way to prevent that. Even if you use encryption somewhere you still need to know how to decrypt it so you can show the messages to the users, so any admin could read the messages.