I've noticed that some sites (including http://jobs.stackoverflow.com) have query strings that look like this:

http://somewebapp.example/?123

as compared to:

http://somewebapp.example/123 or http://somewebapp.example/id/123

What are the reasons that developers choose to implement their web apps' URLs using the first example instead of the second and third examples?

And as a bonus, how would one implement the first example in PHP, given that 123 is the primary key of some row in a database table? (I just need to know how to retrieve 123 from the URL; I already know how to query the database for a primary key of 123.)