..note::

If the legacy application requires it's own session save-handler, do not

override this, rather set ``handler_id: ~``. Note that a save handler cannot

be changed once the session has been started. If the application starts the

session before Symfony initializes the save-handler will have already been

set. You will need ``handler_id: ~``. Only override the save-handler if you

are sure the legacy application can use the Symfony save-handler without

side effects and that the session has not been started before Symfony inializes.

If the legacy application requires its own session save-handler, do not

override this. Instead set ``handler_id: ~``. Note that a save handler

cannot be changed once the session has been started. If the application

starts the session before Symfony is initialized, the save-handler will

have already been set. In this case, you will need ``handler_id: ~``.

Only override the save-handler if you are sure the legacy application

can use the Symfony save-handler without side effects and that the session

has not been started before Symfony is initialized.

For more details, see:doc:`/components/http_foundation/session_php_bridge`.