Saving returned XML data in PHP to an XML file

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I have a php file which requests xml data and displays it it the browser. The XML is displayed by:\[code\]$xmlString = constructPostCallAndGetResponse($endpoint, $item, $postCode, $dist);$resp = simplexml_load_string($xmlString);echo htmlentities($xmlString);\[/code\]How do I save the returned XML to an XML file on my server? Ive read alot about DOM but keep going in circles. Can anyone suggest a simple way of doing this?