The probability of going to each site is defined in advance.
Example:
www.google.com 25%
www.amazon.com 35%
www.microsoft.com 40%
Site users can change the destination / probability of going to a site after the fact by editing urls in their profile. This way if a QR code is already posted somewhere public, it can be changed easily.
This project can be forked to create a custom version of this script, but it will also be available for free here.