Using PHP includes has nothing to do with front or back end, but is a smart way to deal with duplicate code on a website! I do both front and back like many people nowadays.
I have seen more people asking about this on the forum.
Just purchased Pinegrow this week and noticed that PHP includes are not working. Of course I got it working because I also have Mamp Pro, but it would be more convenient if it works strait out of Pinegrow.
Regards & Thanks,