Adding Child element to XML

I have XML of the form

$xml=<<<EOD
<?xml version="1.0" encoding="utf-8"?>
<CompleteSaleRequest xmlns="urn:ebay:apis:eBLBaseComponents">
<RequesterCredentials>
<eBayAuthToken>AUTH_TOKEN_VALUE</eBayAuthToken>
</RequesterCredentials>
<ItemID>ItemIDVALUE</ItemID>
<TransactionID>TransactionIDVALUE</TransactionID>
<Shipped>1</Shipped>
<Paid>1</Paid>
  <Shipment>
    <ShipmentTrackingDetails>
   <ShipmentTrackingNumber>$trackingNo</ShipmentTrackingNumber>
      <ShippingCarrierUsed>UPS</ShippingCarrierUsed>
    </ShipmentTrackingDetails>
    <ShippedTime>2014-09-30 12:41:59.202303</ShippedTime>
    </Shipment>
<ErrorLanguage> string </ErrorLanguage>
EOD;

I want to append more ShipmentTrackingDetails portion using PHP, I need the resulting XML like this:

$xml=<<<EOD
<?xml version="1.0" encoding="utf-8"?>
<CompleteSaleRequest xmlns="urn:ebay:apis:eBLBaseComponents">
<RequesterCredentials>
<eBayAuthToken>AUTH_TOKEN_VALUE</eBayAuthToken>
</RequesterCredentials>
<ItemID>ItemIDVALUE</ItemID>
<TransactionID>TransactionIDVALUE</TransactionID>
<Shipped>1</Shipped>
<Paid>1</Paid>
  <Shipment>
    <ShipmentTrackingDetails>
   <ShipmentTrackingNumber>$trackingNo</ShipmentTrackingNumber>
      <ShippingCarrierUsed>UPS</ShippingCarrierUsed>
    </ShipmentTrackingDetails>

 <ShipmentTrackingDetails>
   <ShipmentTrackingNumber>$trackingNo2</ShipmentTrackingNumber>
      <ShippingCarrierUsed>UPS</ShippingCarrierUsed>
    </ShipmentTrackingDetails>

 <ShipmentTrackingDetails>
   <ShipmentTrackingNumber>$trackingNo3</ShipmentTrackingNumber>
      <ShippingCarrierUsed>UPS</ShippingCarrierUsed>
    </ShipmentTrackingDetails>
    <ShippedTime>2014-09-30 12:41:59.202303</ShippedTime>
    </Shipment>
<ErrorLanguage> string </ErrorLanguage>
EOD;

I am not getting how to do this using php. Please help in getting this working.

Thanks!