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<StaticDataRequest>
   <Header>
      <Code>XXXX</Code>
      <Username>XXXX</Username>
      <Password>XXXX</Password>
   </Header>
   <Body>
      <GetStaticData>countries</GetStaticData>
   </Body>
</StaticDataRequest>

$xml = simplexml_load_file("country_request.xml");
    $xml->Header->Code = 'myCODE';
    $xml->Header->Username  = 'myUsername';
    $xml->Header->Password  = 'myPassword';

<StaticDataRequest><Header><Code>myCODE</Code><Username>myUsername</Username><Password>myPassword</Password></Header><Body><GetStaticData>countries</GetStaticData></Body></StaticDataRequest>

echo (string)$xml;