Why isn't my XML API request accepted by the API? - php

Question

I have been trying, for days, to send a simple XML request to an API. Even with the help of the tech support, absolutely nothing works. I still get an error telling me that my XML isn't well-formed or is invalid.
Here is my cURL request:
$data = array_merge([
'ssl_transaction_type' => "$transactionType",
'ssl_merchant_id' => $this->merchant_id,
'ssl_user_id' => $this->user_id,
'ssl_pin' => $this->pin
], $data);
$xml = new \SimpleXMLElement('<txn/>');
$data = array_flip($data);
array_walk_recursive($data, array ($xml, 'addChild'));
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $this->getXMLUrl());
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, array(
'Content-type: application/x-www-form-urlencoded'
));
curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query(array("xmldata=" . explode(PHP_EOL, $xml->asXML())[1])));
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, false);
$response = curl_exec($ch);
Why do I explode the XML? It's because I need to only get the part with the root, not the version, etc. according to the tech support.
I followed this: XML request is not well-formed or request is incomplete but it still doesn't work.