We run with the configuration option require_previous_session: false,
because our password protected website is accessed by several external
sharepoint crawlers, which do not present their current session cookie
when they log in.

We store sessions in a hosted Memcached service at AWS, which works great.
We use php-memcached to speak with the memcached server.

With PhP5 this setup works fine. We can log in with or without a
preexisting cookie, and sessions are successfully stored in memcached.

With PhP7 we can log in perfectly well with a preexisting cookie, but
not without one, even with identical configuration as we used for PhP5.

When we try to log in without a preexisting cookie on PhP7,NativeSessionStorage.php encounters this error:

PHP Warning:  session_regenerate_id(): Session object destruction failed.
ID: user (path: /var/lib/php/sessions) in 
/var/www/app/vendor/symfony/symfony/src/Symfony/Component/HttpFoundation/Session/
Storage/NativeSessionStorage.php on line 216

For some reason session_regenerate_id looks for a session in the file system which will not exist.

Somehow session_regenerate_id() ignores the session storage configuration we
gave Symfony. (Our session storage configuration in php.ini is not set up)

(If we log in with a preexisting cookie, it gets stored in our Memcached server, and everything works fine)

We use Symfony 2.7.11, Fos Userbundle, Apache/2.4.7 (Ubuntu), PHP 7.0.5-2

From security.yml

    firewalls:
        main:
            pattern: ^/
            form_login:
                provider: fos_userbundle
                use_referer: true
                require_previous_session: false
            logout:    true
            anonymous: true
            simple_preauth:
                authenticator: frontoffice.security.apikey_authenticator

From app/config/config_prod.yml

    framework:
        router:   { resource: "%kernel.root_dir%/config/routing.yml" }
        profiler: { only_exceptions: true }
        session:
            handler_id: session.handler.memcached
            cookie_lifetime: 346896000 

    services:
        memcached:
            class: Memcached
            arguments:
                persistent_id: %session_memcached_prefix%
            calls:
                - [ addServer, [ %session_memcached_host%, %session_memcached_port% ]]

        session.handler.memcached:
            class:     Symfony\Component\HttpFoundation\Session\Storage\Handler\MemcachedSessionHandler
            arguments: [@memcached, { prefix: %session_memcached_prefix%, expiretime: %session_memcached_expire% }]