session storage in database bug (kind of)

I would like to mention that for those of us who store the session data using a PdoSessionHandler the session filesystem won't work.

In this case framework.session.storage_id should be set to session.storage.php_bridge (reference: http://symfony.com/doc/current/cookbook/session/php_bridge.html ).

If I understand correctly this is because Liip uses some deprecated session storage code (see https://github.com/liip/LiipFunctionalTestBundle/issues/80 ). I didn't have time to dig deeper in the Liip code.

In case others try to find the weird exception, when leaving out
storage_id: session.storage.filesystem (i.e. having only handler_id: pdo)
I was getting this exception from the controller I was calling:
Failed to start the session because headers have already been sent by "Project\vendor\phpunit\phpunit\PHPUnit\Util\Printer.php" at line 172.

Reproduce the controller call with handler_id: pdo and inspect the session configuration around framework.session.storage_id, especially the PHP bridge setting. Trace the reported PHPUnit\Util\Printer.php line 172 output and the Liip session setup; done means database-backed sessions start without the headers-already-sent exception.

Written by the indexing model from the issue text.