php-curl-class/examples/upload_file.php

<?php
require __DIR__ . '/vendor/autoload.php';

use \Curl\Curl;

$myfile = curl_file_create('cats.jpg', 'image/png', 'test_name');

$curl = new Curl();
$curl->post('https://httpbin.org/post', array(
    'myfile' => $myfile,
));

if ($curl->error) {
    echo 'Error: ' . $curl->errorMessage . "\n";
} else {
    echo 'Success' . "\n";
}