<a href="some.xml">Downlad XML</a>

header('Content-Disposition: attachment; filename="some.xml"');

<a href="javascript:void(0);" onclick="document.execCommand('SaveAs',true,'some.xml');">Save this page</a>

private function sendXml($xml, $label) {
        ob_clean();
        header('Content-type: text/plain; charset=UTF-8');
        header('Content-Disposition: attachment; filename="' . $label . '.xml"');
        echo ltrim($xml);
        ob_flush();
        exit;
    }

<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom"
   xmlns:creativeCommons="http://backend.userland.com/creativeCommonsRssModule"
   xmlns:re="http://purl.org/atompub/rank/1.0">
<title type="text">How to download an XML without the browser
opening it in another tab - Stack Overflow </title>
...

<a href="http://link/to/file.xml?dummy=dummy" download>Download it!</a>