site:example.com inurl:index.php?id=
Combine these with site:*.edu (educational domains often have old code) or site:*.gov (government legacy systems) to see the scale of the problem. inurl index.php%3Fid=
For the curious security student, inurl:index.php?id= is just the beginning. Once you understand the pattern, you can find specific database columns or CMS versions. site:example