Q:

Where/How does Symfony2 serializes and writes its session data?

To give an overview:
I have an app built on Symfony1 but I'm building the new parts with Symfony2. I've moved the login/logout actions on Symfony2 and made Symfony1 read the session data from Symfony2. By telling Symfony2 to write its session data in the default PHP $_SESSION, it works great, everything is there in arrays and Symfony1 can read the data and login my users automatically.
Now I'm moving the Symfony1 and the Symfony2 apps on their own respective VMs. So instead of writing in PHP $_SESSION, I save the session in a MongoDB (via the MongoDbSessionHandler). But now when I read the session data from the Symfony1 app, I end up with something like this:
_sf2_attributes|a:0:{}_sf2_flashes|a:0:{}_sf2_meta|a:3:{s:1:"u";i:1362655964;s:1:"c";i:1362655964;s:1:"l";s:1:"0";}

and it is definitely not unserializable. Symfony2 seems to serialize the data in its own way and I guess these _sf2_* stuff are the metadatabags. The thing is that I cannot find where this serialization is happening. To be able to unserialize it I need to find how it is serialized. The closest place I've found is in the SessionHandlerProxy:
public function write($id, $data)
{
    return (bool) $this->handler->write($id, $data);
}

the $data passed here contains the serialized data, but I cannot find in the code where it is called.
Any luck?

A:

The MongoDbSessionHandler gets set as the PHP session save handler here: https://github.com/symfony/symfony/blob/master/src/Symfony/Component/HttpFoundation/Session/Storage/NativeSessionStorage.php#L349-L370
session_start() (https://github.com/symfony/symfony/blob/master/src/Symfony/Component/HttpFoundation/Session/Storage/NativeSessionStorage.php#L146) actually reads the contents of the session file/Mongo and create the global $_SESSION variable: http://www.php.net/manual/en/function.session-start.php
Have a look at PHP's session_decode function: http://www.php.net/manual/en/function.session-decode.php - this might be what you're looking for.

