There is a snippet on the RFC for the code: https://wiki.php.net/rfc/curl-file-upload
curl_setopt($curl_handle, CURLOPT_POST, 1);
$args['file'] = new CurlFile('filename.png', 'image/png', 'filename.png');
curl_setopt($curl_handle, CURLOPT_POSTFIELDS, $args);
You can also use the seemingly pointless function curl_file_create( string $filename [, string $mimetype [, string $postname ]] )
if you have a phobia of creating objects.
curl_setopt($curl_handle, CURLOPT_POST, 1);
$args['file'] = curl_file_create('filename.png', 'image/png', 'filename.png');
curl_setopt($curl_handle, CURLOPT_POSTFIELDS, $args);