In Xdebug 3.2, xdebug.client_discovery_header was updated to accept a list of headers to check and fallback to Forwarded For and Remote Address was removed.

However, the documentation continues to say:

If xdebug.client_discovery_header is configured to be a non-empty string, then the value is used as key in the $_SERVER superglobal array to determine which header to use to find the IP address or hostname to use for 'connecting back to'. This setting is only used in combination with xdebug.discover_client_host and is otherwise ignored.

For example, if xdebug.client_discovery_header is set to HTTP_FORWARD_HOST, then Xdebug will check $_SERVER['HTTP_FORWARD_HOST'] before the $_SERVER['HTTP_X_FORWARDED_FOR'] and $_SERVER['REMOTE_ADDR'] variables.

This second paragraph should likely be updated to:

For example, if xdebug.client_discovery_header is set to "HTTP_FORWARD_HOST,HTTP_X_FORWARDED_FOR,REMOTE_ADDR", then Xdebug will attempt to connect to the first host it finds when searching (in order) $_SERVER['HTTP_FORWARD_HOST'], $_SERVER['HTTP_X_FORWARDED_FOR'] and $_SERVER['REMOTE_ADDR'].

Issue that requested change: https://bugs.xdebug.org/view.php?id=1819
PR for change: https://github.com/xdebug/xdebug/pull/841