so by your calling me a "dumb monkey" you went from having someone willing to help, to another person who will just ignore you.
You are what makes the internet rot. I for one will be happy to see you fail. Read More
Im not sure why you want to redirect away from your site 80% of the time, as that would pretty much ensure 80% of the people would not want to visit your site because it's useless.
However, if you wish to pursue this you have 2 options.
1 - pure PHP (with a database of links), and, just as you said, do random(100) and if it's > 80, then echo "href='mylink'" as drawn from database A, and if < 80, then href='fakelink' as drawn from database B.
While I am trying to keep a personal tone out of this, I need to say that pushing 80% of your traffic away is stupid and counter productive... but I'm sure you have your reasons (of which, I have no desire to learn).