index.php

<?php

require __DIR__.'/vendor/.composer/autoload.php';

$app = new Silex\Application(); 

$app->register(new Silex\Provider\SessionServiceProvider());
$app['debug'] = true;

$app->get('/', function() use($app) { 
    $string = 'Hello '.$app['session']->get('last_request_time');
    $app['session']->set('last_request_time', time());
    var_dump($app['session']);
    return $string;
}); 

$app->run();

request #1 to session.php

array(0) {
}
array(1) {
  ["dave"]=>
  string(25) "dave0.01975100 1332462560"
}

request #2 to index.php

object(Symfony\Component\HttpFoundation\Session\Session)#52 (3) {
  ["storage":protected]=>
  object(Symfony\Component\HttpFoundation\Session\Storage\NativeSessionStorage)#53 (4) {
    ["bags":protected]=>
    array(2) {
      ["attributes"]=>
      object(Symfony\Component\HttpFoundation\Session\Attribute\AttributeBag)#56 (3) {
        ["name":"Symfony\Component\HttpFoundation\Session\Attribute\AttributeBag":private]=>
        string(10) "attributes"
        ["storageKey":"Symfony\Component\HttpFoundation\Session\Attribute\AttributeBag":private]=>
        string(15) "_sf2_attributes"
        ["attributes":protected]=>
        &array(1) {
          ["last_request_time"]=>
          int(1332462612)
        }
      }
      ["flashes"]=>
      object(Symfony\Component\HttpFoundation\Session\Flash\FlashBag)#57 (3) {
        ["name":"Symfony\Component\HttpFoundation\Session\Flash\FlashBag":private]=>
        string(7) "flashes"
        ["flashes":"Symfony\Component\HttpFoundation\Session\Flash\FlashBag":private]=>
        &array(0) {
        }
        ["storageKey":"Symfony\Component\HttpFoundation\Session\Flash\FlashBag":private]=>
        string(12) "_sf2_flashes"
      }
    }
    ["started":protected]=>
    bool(true)
    ["closed":protected]=>
    bool(false)
    ["saveHandler":protected]=>
    object(Symfony\Component\HttpFoundation\Session\Storage\Proxy\NativeProxy)#55 (3) {
      ["wrapper":protected]=>
      bool(false)
      ["active":protected]=>
      bool(false)
      ["saveHandlerName":protected]=>
      string(5) "files"
    }
  }
  ["flashName":"Symfony\Component\HttpFoundation\Session\Session":private]=>
  string(7) "flashes"
  ["attributeName":"Symfony\Component\HttpFoundation\Session\Session":private]=>
  string(10) "attributes"
}
Hello 

request #3 to session.php

array(3) {
  ["dave"]=>
  string(25) "dave0.01975100 1332462560"
  ["_sf2_attributes"]=>
  array(1) {
    ["last_request_time"]=>
    int(1332462612)
  }
  ["_sf2_flashes"]=>
  array(0) {
  }
}
array(3) {
  ["dave"]=>
  string(25) "dave0.11753100 1332462642"
  ["_sf2_attributes"]=>
  array(1) {
    ["last_request_time"]=>
    int(1332462612)
  }
  ["_sf2_flashes"]=>
  array(0) {
  }
}

session.php

<?php

session_start();
var_dump($_SESSION);
$_SESSION['dave'] = 'dave' . microtime();
var_dump($_SESSION);