I feel that you are taking the wrong route, because you ignore numerous suggestions from people on this forum to fix your PHP code, but here's how to disable PHP scripts in myfolder:

location ~ ^/myfolder/.*\.php$ {
return 403;
}

Which forum software are you running? Perhaps, changing the forum application will fix all of your problems without the need to alter nginx confoguration. Ask yourself why remote file inclusion is ever necessary and why is it there in the first place?

Andrejs