How to gzip XML file? - php

Question

I am working on integrating my application with the API of another application. Previously I have done such integrations using JSON and had no problem, unfortunately, the application I want to connect to uses XML which I have not interacted with before.
The task is to load an .xml file into a PHP script, then perform gzip on that XML, and then encode the byte array to base64.
I have the XML file ready, I can use base64_encode() too, but I sit on this gzip for several hours and nothing works. (I load the XML file into the script using simplexml_load_file)
I tried using gzencode, gzcompress but it doesn't work.
Authentication involves combining HTTP parameters and encoding them with urlencode(). The req_sig (signature) parameter contains the encoded parameters (mentioned above) + API code. If everything matches then a response is returned, if not then an error is displayed: ERROR4000Signature does not match, even though the signature matches, because it is generated the same way in other queries using GET.
My code:
$xml=simplexml_load_file("request.xml") or die("Failed to load");
$gzip = gzencode($xml);
$base64 = base64_encode($gzip);
$username = 'test-app-user';
$req_id = rand(10000000000, 99999999999);
$api_token = "za5rth23ku776fdvbn2";
$string_bazowy = "command=$base64"."req_id=$req_id"."username=$username";
$encoded = urlencode($string_bazowy);
$string_zmieniony = "$encoded"."$api_token";
$md5 = md5($string_zmieniony);
$curl = curl_init();
curl_setopt_array($curl, array(
CURLOPT_URL => "https://saldeo-test.brainshare.pl/api/xml/1.0/company/synchronize?command=$base64&req_id=$req_id&username=$username&req_sig=$md5",
CURLOPT_RETURNTRANSFER => true,
CURLOPT_ENCODING => '',
CURLOPT_MAXREDIRS => 10,
CURLOPT_TIMEOUT => 0,
CURLOPT_FOLLOWLOCATION => true,
CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
CURLOPT_CUSTOMREQUEST => 'POST',
));
$response = curl_exec($curl);
curl_close($curl);
echo $response;