Use mamcache session with Facebook PHP SDK 4

We are using on our current website memcache for the session and an old extended version of the Facebook SDK 3.1.1 that can handles memcache-session.

Now we have to update to SDK V4, but i can't find an example, how to use memcache to store the FB-Sessions instead of the normal §_SESSION the SDK 4 is using.

Any clue? Thanks!