Bridging the gap between standard $_SESSION and HttpFoundation Session with Silex.

index.php

<?php
// Register all other Silex Providers

$app->register(new Silex\Provider\SessionServiceProvider, array(
    'cookie_lifetime' => 86400
));

/**
 * LegacyStorage for setting Base SESSION keys into Symfony Session
 */
$app['session.storage'] = $app->share(function ($app) {
    return new Session\Storage\LegacySessionStorage();
});
$app['session']->start();

LegacySessionStorage.php

<?php

namespace Session\Storage;

use Symfony\Component\HttpFoundation\Session\Storage\PhpBridgeSessionStorage;

/**
 * Session storage that avoids using _sf2_attributes subkey in the $_SESSION
 * superglobal but instead it uses the root variable.
 *
 * @link(GIST, https://gist.github.com/tcz/9756112)
 */
class LegacySessionStorage extends PhpBridgeSessionStorage
{
    const SYMFONY_SESSION_STORAGE_KEY  = '_sf2_attributes';
    const SYMFONY_SESSION_FLASH_KEY    = '_sf2_flashes';
    const SYMFONY_SESSION_METADATA_KEY = '_sf2_meta';

    /**
     * @inheritdoc
     */
    protected function loadSession(array &$session = null)
    {
        if (null === $session) {
            $session = &$_SESSION;
        }

        parent::loadSession($session);

        foreach ($this->bags as $bag) {
            $key = $bag->getStorageKey();

            if (self::SYMFONY_SESSION_STORAGE_KEY === $key)
            {
                $bag->initialize($session);
            }
        }
    }
}