I don't see any purpose in this other than to either look for open source applications to exploit in the wild or to just pick on a really easy target to make us all feel superior.You could do a similar search for common anti-patterns in almost every language you can think of, so this is not specific to PHP, just perhaps a bit more common due to the low-bar for picking up PHP.
A useful aim of this sort of post to me is to underline that people should not do this in a script exposed to the internet, and if people read comments on searches like this, they'll find out why. I don't think it's appropriate to use variables in query strings even for learners as it's teaching bad habits - I have personally seen scripts go into production based on bad examples at a former place of work (in perl, not php), allowing spammers to use a contact form. You can guarantee that for each of these common vulnerabilities there are scripts in the wild scanning websites and trying to exploit them.
A surprising number of people still do make mistakes like this, so it does more good than harm to point that out I think, and certainly doesn't make me feel superior, it makes me wonder which vulnerabilities I've missed in my own code.