It's not script, it's custom error pages.
When visitor come to your site and search for something that not valid, he gets error pages, every visitor each visit get at least 1 error page, now you can earn from these pages, really good money maker.
Only need to edit 1 PHP file with your advertisements code.
Revenue Sources (For the owner)
You have multiple revenue sources: Adsense, Adbrite, ClickSor, CPALead, etc.