Remember that varchar normally limits to 255 characters although MySQL has alrger VARCHAR options available. Personally I would probably use a MEDIUMTEXT field to store the XML as it gives me up to 16MB of data.
To convert the XML from an object to a string use the ->asXML method or just store it directly from your $_POST data
$querystring = "INSERT INTO MYTABLE(xml) VALUES ('{$_POST['xml']}')";
or
$querystring = "INSERT INTO MYTABLE(xml) VALUES ('{$xml->asXml()}')";