Yes, sorry, I was not very clear...
I have two quite different cases:
1) a database of similar items
referenced through widget.php?id=number
So, in the text of my pages (that will be crawled) I put links in the form "widget.php?id=number" that, after all, is a GET request.
These are all distinct pages, spidered differently and adsensed differently
2) a set of operations that the user can do on those items
referenced through widget.php?id=number&x=123&y=456
the page is the same but it takes several other parameters
These pages will never get spidered (they are called by a form, after all) and due to their highly variable url will never get proper ads
In this case I changed the form method to POST, so the same page is called just "widget.php"
No spidering anyway, but at least a set of default ads
The best part is that due to php's flexibility, the code of widget.php hardly needs to be changed!