Creating XML string with multiple items in PHP

I'm trying to create an XML string in PHP, with a loop. But I get a parse error.

This is the XML string I'm trying to create:

$xmlstr = <<<XML
<?xml version='1.0'?>
<DOCUMENT>;

while ($stmt->fetch())
{ 
  $xmlstr.= 
             <ORDER>
               <ORDERNUMBER>$ordernumber</ORDERNUMBER>
               <USERNAME>$username</USERNAME>
               <EMAIL>$mail</EMAIL>
               <STATUS>$status</STATUS>
               </ORDER>;
           };

  $xmlstr.= 
  </DOCUMENT>
  XML;