mySQL output to XML error

I want to show my database in XML if the API-key is correctly. Everything works fine until the XML page should show up. It shows this error:

This page contains the following errors:

error on line 3 at column 1: Extra content at the end of the document
Below is a rendering of the page up to the first error.

My code:

                $sql = "SELECT * FROM WKplayers";
                $res = mysqli_query($con, $sql);

                $xml = new XMLWriter();

                $xml->openURI("php://output");
                $xml->startDocument();
                $xml->setIndent(true);

                $xml->startElement('Players');

                while ($row = mysqli_fetch_assoc($con, $res)) {
                  $xml->startElement("Player");

                  $xml->writeRaw($row['Player']);

                  $xml->endElement();
                }

                $xml->endElement();

                header('Content-type: text/xml');
                $xml->flush();