Almost without exception, today’s Internet users prove their identity online using a fixed account name and password. In the past, this simple system provided sufficient security. But with the growing popularity of online banking and eCommerce, the value of stealing online identities has skyrocketed. And the increasing presence and “spyware” and “malware” on innocent users’ computers means that users can be “watched” while logging onto their banking and other eCommerce sites. Once their logon credentials have been “captured” and stolen, Internet criminals can easily assume their identity.

The trouble with a username and password is that they never change. We create them, write them down or memorize them, then use them over and over again. What has been needed is an inexpensive system that provides something which changes everytime it is used. GRC’s Perfect Paper Passwords system offers a simple, safe and secure, free and well documented solution that is being adopted by a growing number of security-conscious Internet facilities to provide their users with state-of-the-art cryptographic logon security.

And…  this PPP implementation can even be done in PHP [direct link], although the authors page is broken at the moment.  An alternate author is Daniel Hodder, his page appears to be working at first blush.