Securing your encryption key

[eluser]bikuta[/eluser]
I am writing an application that needs to have very high security in terms of data protection. I was thinking maybe I could use the encrypt CI library, however I'm not sure how I can protect the key.

Is there anyway to protect the key if someone gains access to the server? I didn't think there would be, but I could be wrong.

What would be the most secure way to store the key?

Is there a way to encrypt the php script, i.e., to protect the application source code? so that way even though they can read it, they would not be able to understand it.