PHP automatically adds HTTP_ in front of HTTP header names when storing them in the $_SERVER array. In your case, that means that the FORWARD_HOST header that you request with cURL ends up having to be HTTP_FORWARD_HOST as xdebug.client_discovery_header value. This should therefore work if you set:

xdebug.client_discovery_header = HTTP_FORWARD_HOST

Let me know if that does the trick?

The documentation is indeed unclear about this, and I will update this.

Sadly it does not do the trick but helped me a step forward ;)

FORWARD_HOST was not in var_dump($SERVER). Looks like you need to use different variables. Also there is a weird thingy about using - in the request and it gets converted to `but using_` directly is not working.
So anyway the working version is to use:
curl -H 'XDEBUG_IP: 1.1.1.1' ....
and in the xdebug config
xdebug.client_discovery_header = HTTP_XDEBUG_IP

Hope this will safe someone else time ;)

thanks for your support derick

I have updated the documentation for this setting and added a warning, to explain this situation: https://xdebug.org/docs/all_settings#client_discovery_header