Async Upload Using Curl in PHP - curl_multi_exec()

I'm trying to figure out for days how would it be possible if at all, to upload multiple files parallel using PHP.

given I have a class called Request with 2 methods register() and executeAll():

class Request
{
    protected $curlHandlers = [];
    protected $curlMultiHandle = null;

    public function register($url , $file = []) 
    {
        if (empty($file)) {
            return; 
        }

        // Register the curl multihandle only once.
        if (is_null($this->curlMultiHandle)) {
            $this->curlMultiHandle = curl_multi_init();
        }

        $curlHandler = curl_init($url);

        $options = [
            CURLOPT_ENCODING => 'gzip',
            CURLOPT_FOLLOWLOCATION => true,
            CURLOPT_RETURNTRANSFER => true,
            CURLOPT_POST => 1,
            CURLOPT_POSTFIELDS => $file,
            CURLOPT_USERAGENT => 'Curl',
            CURLOPT_HTTPHEADER => [
                'Content-Type' => 'multipart/form-data; boundry=-------------'.uniqid()
            ]
        ];

        curl_setopt_array($curlHandler, $options);

        $this->curlHandlers[] = $curlHandler;
        curl_multi_add_handle($this->curlMultiHandle, $curlHandler);
    }

    public function executeAll() 
    {
        $responses = [];

        $running = null;

        do {
            curl_multi_exec($this->curlMultiHandle, $running);
        } while ($running > 0);

        foreach ($this->curlHandlers as $id => $handle) {
            $responses[$id] = curl_multi_getcontent($handle);
            curl_multi_remove_handle($this->curlMultiHandle, $handle);
        }

        curl_multi_close($this->curlMultiHandle);

        return $responses;
    }
}

$request = new Request;

// For this example I will keep it simple uploading only one file.
// that was posted using a regular HTML form multipart
$resource = $_FILES['file'];
$request->register('http://localhost/upload.php', $resource);

$responses = $request->executeAll(); // outputs an empty subset of array(1) { 0 => array(0) { } }

Problem:

Can't figure out why on upload.php (the script which is my endpoint url on the register method) $_FILES is always an empty array:

upload.php:

<?php

    var_dump($_FILES); // outputs an empty array(0) { }

Things I've already tried:

prefixing the data with @, like so:

    $options = [
        CURLOPT_ENCODING => 'gzip',
        CURLOPT_FOLLOWLOCATION => true,
        CURLOPT_RETURNTRANSFER => true,
        CURLOPT_POST => 1,
        CURLOPT_POSTFIELDS => ['file' => '@'.$file['tmp_name']],
        CURLOPT_USERAGENT => 'Curl',
        CURLOPT_HTTPHEADER => [
            'Content-Type' => 'multipart/form-data; boundry=-------------'.uniqid()
        ]
    ];

That unfortunately did not work.

What am I doing wrong ?

how could I get the file resource stored in $_FILES global on the posted script (upload.php) ?

Further Debug Information:

on upload.php print_r the headers I get as response the following:

array(1) {
  [0]=>
  string(260) "Array
(
    [Host] => localhost
    [User-Agent] => Curl
    [Accept] => */*
    [Accept-Encoding] => gzip
    [Content-Length] => 1106
    [Content-Type] => multipart/form-data; boundary=------------------------966fdfac935d2bba
    [Expect] => 100-continue
 )
"
}

print_r($_POST) on upload.php gives the following response back:

array(1) {
  [0]=>
  string(290) "Array
(
    [name] => example-1.jpg
    [encrypted_name] => Nk9pN21IWExiT2VlNnpHU3JRRkZKZz09.jpg
    [type] => image/jpeg
    [extension] => jpg
    [tmp_name] => C:\xampp\tmp\php77D7.tmp
    [error] => 0
    [size] => 62473
    [encryption] => 1
    [success] => 
    [errorMessage] => 
)
"
}

I appreciate any answer.

Thanks,

Eden