Only a few INI settings for Xdebug are supported through XDEBUG_CONFIG. remote_connect_back isn't one of them. What make you think (documentation, for example) that this was supported?

Well, documentation says:

When running the script from the command line you need to set an environment variable, like:
export XDEBUG_CONFIG="idekey=session_name"
php myscript.php
You can also configure the xdebug.remote_host, xdebug.remote_port, xdebug.remote_mode and xdebug.remote_handler in this same environment variable as long as you separate the values by a space:
export XDEBUG_CONFIG="idekey=session_name remote_host=localhost profiler_enable=1"
All settings that you can set through the XDEBUG_CONFIG setting can also be set with normal php.ini settings.

I thought, that if documentation says, that xdebug.remote_host, xdebug.remote_port, xdebug.remote_mode and xdebug.remote_handler can be set in XDEBUG_CONFIG, but also profiler_enable usage mentioned in example I can set something else in XDEBUG_CONFIG, for example remote_connect_back (that was extremely useful in my situation).

But why, for god sake, this parameter was not supported? I can understand, that whatever trace_options is not so important for that, but remote_connect_back should be there.

Hi!

Xdebug has over 50 settings, and each of them would need to be hardcoded in
the xdebug_env_config function in xdebug.c to support this. As PHP already
allows configuration through passing -doption=value on the command line, there
must be a large advantage for the setting to be configurable through the
XDEBUG_CONFIG environment variable for me to want to add it there.

The original intention for allowing configuration settings to be made through
an environment variable was to allow IDEs to set up a known state to allow
for debugging. This got later extended to triggering profiling and tracing
as well. As xdebug.remote_connect_back puts the debugger in an unstable
state (as the IP address to connect to can change away from what the IDE
expects), this setting was not included.

cheers,
Derick

Excuse me, but i have to reopen it to clarify, why this feature is important for my situation.
I have docker container with php+xdebug in it. Xdebug configured with remote_connect_back=1 in php.ini and when i start container on Linux machine all is working. But when i start it on Windows machine, there is a problem between Hyper-v virtual adapter and docker network, well, xdebug cannot refer to docker adapter's IP, because tcp port is not forwarded via Hyper-v. So the only way to make it work - is to somehow set xdebug remote_connect_back=0 and hardcode remote_host to host IP (developer just need to set env variable XDEBUG_CONFIG=192.168.0.23, where 192.168.0.23 - his machine's IP). Well, container can take env variable value from host machine and use it inside itself.
Yep, this is Windows specific issue and the main goal was to make windows developers (they are in minority) in our company to do some workaround instead of linux users (because we can make all users to set XDEBUG_CONFIG=192.168.0.x and it will work).

Makes sense. I've added it to 2.5.0RC2.