使用PHP和cURL使用HTTPS身份验证发布XML

Question

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, $requestXML);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);

This is working pretty straight forward with a test xml string.

$requestXML = "<?xml version='1.0' encoding='utf-8'?><request>
<id>12345</id>
<email>[e-mail]</email><request>";

(https://magp.ie/2010/04/12/post-xml-with-https-authentication-using-php-curl/)

I am combining this with this:

function xml_encode($mixed, $domElement=null, $DOMDocument=null) {
    if (is_null($DOMDocument)) {
        $DOMDocument = new DOMDocument('1.0', 'utf-8');
        $DOMDocument->formatOutput = true;
        xml_encode($mixed, $DOMDocument, $DOMDocument);
        echo $DOMDocument->saveXML();
    }
    else {
        if (is_array($mixed)) {
            foreach ($mixed as $index => $mixedElement) {
                if (is_int($index)) {
                    if ($index === 0) {
                        $node = $domElement;
                    }
                    else {
                        $node = $DOMDocument->createElement($domElement->tagName);
                        $domElement->parentNode->appendChild($node);
                    }
                }
                else {
                    $plural = $DOMDocument->createElement($index);
                    $domElement->appendChild($plural);
                    $node = $plural;
                    if (rtrim($index,'s')!==$index && count($mixedElement)>1) {
                        $singular = $DOMDocument->createElement(rtrim($index, 's'));
                        $plural->appendChild($singular);
                        $node = $singular;
                    }
                }

                xml_encode($mixedElement, $node, $DOMDocument);
            }
        }
        else {
            $domElement->appendChild($DOMDocument->createTextNode($mixed));
        }
    }
}

to generate the xml from an array (saving it to $requestXML) (https://www.darklaunch.com/2009/05/23/php-xml-encode-using-domdocument-convert-array-to-xml-json-encode)

It works with the test xml, but unfortunately not with generated XML. I concat the $requestXML like this:

$requestXML = "&data=";$requestXML .= xml_encode($data); // if the test xml comes here, everything works well

I guess the generated xml is not really a string, so that's maybe it is not send with the POSTFIELDS?