• First, get to know who is from search engines by checking the referers. There are so many search engines to list that I don't want to match their names into the referers' url. Instead, I thought "?" exists in all these query-like engine search urls. But I saw some "?" is inside some url from msn/live spaces as well. So I switched to check whether "&" is inside or not.

• Second, keep the information that the visitor is coming from searching if the visitor finds some other interesting posts within the site and go around, so google ads can be continuously displaced. Since the varible, which keeps the information, needs to go through pages, then it is best to be stored in session. So I do the following

• Third, how about then someone is willing to see the ads and they are not from search engines (who will be that? me, at least, at my own site :p ). So just put another tricky mark in the url to pass the variable to $_GET and then keep the option remembered by session. You may notice from the code, using "?ad=" at the end of url will let you just have google ads once by "?ad=(anything except y)" or for the whole session by "?ad=y"

I didn't use this method here for this blog, but you do can test it at this journal blog here:

Directly use this link: http://www.ohehlium.com/journal/2006/10/01/140/ (no adsense)

Then try google first, and see: Nobel site:ohehlium.com, click this link and go, you will have ads there and you will have it when visiting around unless you close the browser

Or try this link: http://www.ohehlium.com/journal/2006/10/01/140/?ad=1, you will have one time adsense

Or then this one: http://www.ohehlium.com/journal/2006/10/01/140/?ad=y, then the same thing as from search engines