Follup of this comment:

Something is missing since a long time in the configuration of the prod environment for error handling: the ErrorsLoggersListener does a ErrorHandler::setLogger(), but ErrorHandler::register() is never called, so no logging can ever happen, except the one done by the native PHP handler.

This led me to two points:

• I think we should always call ErrorHandler::register(), and use set_error_handler()'s second argument to catch only what we want to catch (thus no perf impact in prod).
• Does Symfony have recommended settings for log_errors and error_log ini settings? Because IMHO it could be a good idea to put native php logs in app/logs/, if the settings are not configured.