But is it because C programmers make more ugly hacks or that C programmers are more likely to know when they are making an ugly hack?

I think PHP is more or less just for the web, giving C programmers more ugly hack surface overall.

Also PHP itself is an ugly hack of C anyway.

There is a bias because there is more C code. The number of repositories doesn't equal to the quantity of code and if you search for general things that are used in both languages frequently then there is always more C results, for example:

https://github.com/search?p=1&q=if+&ref=searchresults&type=C...

https://github.com/search?p=1&q=return+&ref=searchresults&ty...

Fair enough. If there is a bias towards C I would guess it was more due to the situations in which C gets used, than C itself. Talking to a friend who has to try and get the same C/C++ codebase to compile for 6 different platforms (consoles) has been an eye opening experience. That kind of environment is like a slightly warm petri dish for ugly hacks.