IIS 10 - Setting up PHP Application behind Reverse Proxy

Question

I am currently struggling to get the right config in IIS, as I am normally using Unix servers and nginx.

I have a reverse proxy (nginx) that forwards all requests to subdomain.domain.com/xxx to my IIS server.

Here I have a website that hosts a PHP application. I created a virtual directory xxx. Without this virtual directory, subdomain.domain.com is treated as root, and IIS look for directory xxx (which does not exist).

I have pointed both the website and the virtual directory to the same physical path. I then get an error, because one config is written twice to the web.config.

If I change the physical directory of the website, the application won`t work, because it uses the websites physical directory in its configuration.

Any help as to how this should be configured is greatly appreciated, I am starting to lose my mind in all this UI boxes of IIS.

Relevant part of the nginx conf:

location /spc {
    proxy_set_header Host $host;
    proxy_set_header   X-Real-IP          $remote_addr;
    proxy_set_header   X-Forwarded-Proto  $scheme;
    proxy_set_header   X-Forwarded-For    $proxy_add_x_forwarded_for;
    proxy_set_header   X-Forwarded-Host   $host;
        proxy_pass https://xx.xx.de:9443;
}