LegacySessionStorage.php

<?php

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

/**
 * Session sotrage that avoids using _sf2_attributes subkey
 * in the $_SESSION superglobal but instead it uses
 * the root variable.
 */
class LegacySessionStorage extends NativeSessionStorage 
{
	const SYMFONY_SESSION_SUBKEY = '_sf2_attributes';

	/**
     * @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_SUBKEY === $key)
            {
            	$bag->initialize($session);
            }
        }
    }
}