/**
  * unset一个bo
  * @param unknown_type $sessionId
  * @return boolean
  */
 public static function unsetInstance($sessionId = null)
 {
     if ($sessionId == null) {
         self::$_bo = null;
     }
     if (!isset(self::$_bo[$sessionId])) {
         return false;
     }
     self::$_bo[$sessionId] = null;
     return true;
 }