How to get Shipment Increment ID by Order ID in Magento

Hello Guys. Can someone tell me how to get the shipment increment id by order id in Magento ?

I need this because I use an outside php file to add tracking information to a shipment and it needs the shipment id for this.

Thank you for all your help.

I am using the code below to add tracking info

$shipmentIncrementId='300000002';
$trackNumber='123456';
$carrier='custom';
$title='server10';


$shipment = Mage::getModel('sales/order_shipment')->loadByIncrementId($shipmentIncrementId);

         /* @var $shipment Mage_Sales_Model_Order_Shipment */



         $track = Mage::getModel('sales/order_shipment_track')
                     ->setNumber($trackNumber)
                     ->setCarrierCode($carrier)
                     ->setTitle($title);

         $shipment->addTrack($track);

         try {
             $shipment->save();
         } catch (Mage_Core_Exception $e) {
             $thiss->_fault('data_invalid', $e->getMessage());
         }

         return $track->getId();

print_r($shipment);