/**
  * Returns the singleton instance of Session
  */
 public static function getInstance()
 {
     if (!session_id()) {
         session_start();
     }
     // Default object is the one stored in the $_SESSION-variable (if existant)
     $object_registered = isset($_SESSION[self::$SESSION_IDENTIFIER]);
     $session = Singleton::getSingleton(get_class(), $object_registered ? $_SESSION[self::$SESSION_IDENTIFIER] : null);
     if (!$object_registered) {
         //save singleton object in session:
         $_SESSION[self::$SESSION_IDENTIFIER] = $session;
     }
     return $session;
 }
 public static function getInstance()
 {
     return Singleton::getSingleton(get_class());
 }