Q:

How do you start a memcached session in symfony2?

I'll try to be as detailed as possible because I was searching for now , 75 hours for a solution ..
Brace yourselves.. here we go :
I'm trying to implement the MemcacheD session handler for Symfony2 :
I have downloaded the necessary libraries and then configured Symfony2 as follows:
In app/config.yml :
imports:
    # ....
    - { resource: services/session.yml }

framework:
    # ....
    session:
      handler_id:     session.handler.memcached

app/config/parameters.yml:
session_memcached_host:     127.0.0.1
session_memcached_port:     11211
session_memcached_prefix:   ng_
session_memcached_expire:   43200

app/services/session.yml : 
services:
    session.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: [@session.memcached, { prefix: %session_memcached_prefix%, expiretime: %session_memcached_expire% }]

My biggest Question so far is : How do you start a session ?
Normally, you would have $session = new Session();
but not for handlers, since the documentation states (code converted to Memcached): 
$storage = new NativeSessionStorage(array(), new MemcachedSessionHandler());
$session = new Session($storage); 

and this is some really weird, because the constructor needs a Memcached instance for argument, which is not given in the example of the official docs
What I did was to get the instance from the service running :
$memcached = $this->get('session.memcached');
$storage = new NativeSessionStorage(array(), new MemcachedSessionHandler($memcached));
$session = new Session($storage); 

This didn't throw any exception but then again, so I filled the session with data:
$session->set('userName', $name);
$session->set('userfName', $fname);
$session->set('userPass', $pass);
$session->set('userId', $userId);
$session->set('userPost', $userPost);
$session->set('userImage', $userImage);
$session->set('logged', true);

Everything is perfect? Wait for it...I go on another controller and run the following: 
$session = $this->get('session');
var_dump($session->get('userId')); // returns null

This means that either the session was not persisted (Memcached log says otherwise) or I don't do it right, which leads to my second question: How do I read sessions from the Memcached Server?
Please, I really need this to work because I need them in websockets project.
Following the comments this is what I did: 
app/config/config.yml:
session:
    cookie_lifetime: 43200
    # handler_id set to null will use default session handler from php.ini
    handler_id:  session
    save_path: ~

app/config/services.yml:
services:
    session.memcached:
        class: Memcached
        arguments:
            persistent_id: %session_memcached_prefix%
        calls:
            - [ addServer, [ %session_memcached_host%, %session_memcached_port% ]]

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

The error I get: 

PHP Catchable fatal error:  Argument 1 passed to Symfony\Component\HttpFoundation\Request::setSession() must implement interface Symfony\Component\HttpFoundation\Session\SessionInterface, instance of Symfony\Component\HttpFoundation\Session\Storage\Handler\MemcachedSessionHandler given

A:

Symfony full stack framework
If you use the full stack Symfony framework, than configuring memcached as a session handler is as simple as specifying it in your php.ini:
session.save_handler=memcached
session.save_path=localhost:11211

Make sure that the handler_id is set to null (~) in your app/config/config.yml. This way Symfony will use the native php handler:
framework:
    session:
        # handler_id set to null will use default session handler from php.ini
        handler_id:  ~

Now you can start using your session. It is going to be stored in memcached.
Accessing the session
Session can be accessed from the Request object:
$request->getSession()->set('name', 'Kuba');

HttpFoundation component
The same can be set up outside of a full stack framework with the HttpFoundation component alone:
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Session\Session;
use Symfony\Component\HttpFoundation\Session\Attribute\NamespacedAttributeBag;
use Symfony\Component\HttpFoundation\Session\Storage\NativeSessionStorage;

$storage = new NativeSessionStorage(array());
$session = new Session($storage, new NamespacedAttributeBag());

$request = Request::createFromGlobals();
$request->setSession($session);

The above snippet sets the session up just like the full stack framework does (by default). This way, the script you'll put this code in, will share the session with a full Symfony application.

