I am writing an admin section for a project I am working on...

I was just wondering if anyone can think of unique ways to authenticate someone in PHP? I am trying to think past the password.

Ideas I have include requiring having a certain IP to enter an admin section, but my creativity is limited by my knowledge of PHP. I'm hoping others have possibly ran into some creative ways a user can authenticate themselves over the web (w/o hardware or secure ID's or the like)

Thanks!