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Unknown entity changing ini settings in Unknown file on line 0


How do I go about debugging and eventually solving this error message
I keep getting on my site?

> PHP Warning: Unknown: Headers already sent. You cannot change the session module's ini settings at this time in Unknown on line 0

I cannot reliably reproduce it, but it happens several times a day on
the production environment, and it has also happened on my local
environment (but far less frequently).

The related access_log entries do not help (at least not
straightforwardly) – I do not see anything apparently common to all
the suspicious / related requests.
I've checked for ini and session related functions used in the code,
and I couldn't see a problem there. Besides, if there was an explicit
ini_set, header, session_start or similar problem in the code, the
error message would be more generous with the trace than just "Unknown
in Unknown on line 0", right?

I've tried setting "output_buffering = On", which may be reducing the
amount of warnings (or it may be a coincidence), but they are still

The Nginx and PHP configuration syntax seems to be all right, although
we may be missing something there.

PHP version is 7.2.17
We're using Nginx and PHP-FPM.

The modules are loaded by the /etc/php/7.2/mods-available convention,
and there seem to be no modules being loaded more than once.
We also have XDebug module enabled at the moment, but the warning
appeared also when it was disabled.

Any help would be much appreciated.