XML data is formatted text so, assuming that have the XML text in a PHP variable called, say, $xmltext, you can Post it in a textarea, something like:
<form action="url" method="post">
<textarea rows="99" cols="127" name="xmltext" readonly><?php echo $xmltext;?></textarea>
<input type="submit" value="submit">
Note: I have called the POST parameter "xmltext", but you would have to use whatever name the recipient code expects the XML data to be passed in, together with whateve additional data is required (if any).
P.S. There is probably a much better way of doing it, I just offer this as a thought...