This is in line with the documentation. Only some settings are settable through XDEBUG_CONFIG, as described at https://xdebug.org/docs/all_settings#XDEBUG_CONFIG. The https://xdebug.org/docs/all_settings#start_with_request documentation does not state it can be set there.

What is your specific use case here?

Ideally, PHP itself would have functionality to do this, so that I don't have to hack it for Xdebug...

The https://xdebug.org/docs/all_settings#start_with_request documentation does not state it can be set there.

Oh I must have missed that, apologies!

What is your specific use case here?

The use-case here is that we run multiple (identical) Docker containers of a PHP application. For validation and testing purposes we want the containers to use identical configuration (ini files, extensions, etc).

For development we want to enable debugging (in various modes and configurations), so we do that by setting XDEBUG_MODE and XDEBUG_CONFIG environment variables for the various containers. It would be really useful (for us at least) if we could set the trigger mode via an environment variable, so that we do not have to build a dedicated container just to set that one configuration.