To get soap:Header
with it's children elements
// load full xml
$xml = simplexml_load_string($s);
// get all namespaces used in doc by seting true and register soap for xpath
$xml->registerXPathNamespace ('soap', $xml->getDocNamespaces(true)['soap']);
// get the part you want
$myxml = $xml->xpath('/env:Envelope/env:Body/soap:Header');