Description:
------------
When storing an object of a singleton class in the session, you can have more than one instance of that class in other subsequent executions.

Reproduce code:
---------------
class Singleton {

	private static $instance;
	
	private function __construct() {}
	
	final public static function getInstance()
	{
		if (!isset(Singleton::$instance))
		{
			Singleton::$instance = &new Singleton;	
		}
		return Singleton::$instance;
	}

}
session_start();

if(!isset($_SESSION['singleton'])) $_SESSION['singleton'] = Singleton::getInstance();

$test = Singleton::getInstance();
$anotherTest = Singleton::getInstance();

var_dump(Singleton::getInstance() === $test);
var_dump(Singleton::getInstance() === $anotherTest);
var_dump($test === $anotherTest);

var_dump(Singleton::getInstance() === $_SESSION['singleton']);
var_dump($test === $_SESSION['singleton']);
var_dump($anotherTest === $_SESSION['singleton']);

session_write_close();

Expected result:
----------------
Session not started (first execution)

bool(true)
bool(true)
bool(true)
bool(true)
bool(true)
bool(true)

Subsequent executions

bool(true)
bool(true)
bool(true)
bool(true)
bool(true)
bool(true)

Actual result:
--------------
Session not started (first execution it's OK)

bool(true)
bool(true)
bool(true)
bool(true)
bool(true)
bool(true)

Subsequent executions (fails, multiple instances)

bool(true)
bool(true)
bool(true)
bool(false)
bool(false)
bool(false)