1. Get machine fingerprint based on hardware id of the devices. Use your judgment on which one you want to include. 2. Create a one way hash, with a salt. Store it locally. 3. Call home with this key to activate the product. 4. Every once in a while check the finger print again to see if it has changed if it has redirect to the activation page. This would happen only if the hardware changed.

I am assuming that the people won't direct access to this logic in your PHP application. I don't use PHP so don't know if you can pre compile your app.