No callback from API after php cURL request - php

Question

My code makes a curl request to an API that converts image formats e.g. png to jpg.
The API documentation offers a callback from the API which, when the conversion is finished, will send a GET request to a url on my server (hosted, not localhost). I provide this url to the API with the key/value pair:
"callback" => "12coins.net/cc_callback.php"
Unfortunately the API never calls back. Are my curl_setopt parameters wrong or what could be the problem?
$ch_start_process = curl_init();
$start_process_data = array(
"callback" => "https://12coins.com/cc_callback.php",
"input" => "download",
"file" => "https://12coins.com/photo_file.png",//the image I want converted
"tag" => "tag - unused for now",
"outputformat" => "jpg");
$process_url = "https:".$url_from_create;//prepend https to construct a valid endpoint.
//$url_from_create is a url returned by the API to a request immediately prior to this one
curl_setopt($ch_start_process, CURLOPT_URL, $process_url);
curl_setopt($ch_start_process, CURLOPT_POSTFIELDS, http_build_query ($start_process_data));
curl_setopt($ch_start_process, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch_start_process, CURLOPT_IPRESOLVE, CURL_IPRESOLVE_V4 );
$start_response=curl_exec($ch_start_process);//assign return value of curl_exec()
This is the script on my (remote) server 12coins.net/cc_callback.php to which the API should call back but doesn't:
<?php
header('Access-Control-Allow-Origin: *');
echo 'cc_callback has been called';
echo 'The GET request from CloudConvert is: '. $_GET;
?>

Answer 1

The API does in fact make a call back. It was just that with the code as shown in cc_callback.php there was no way for me to detect the callback. I had assumed that the echo statements would allow me to see the API's response in the (Chrome) network tab of my brower's dev tools. But of course the echo statements echo to the client that 'called' it with a GET request. In this case, that client is the API and not my browser/html page.
Realising this, I was then easily able to check that it had worked all along by adding
mail(myemailaddress#gmail.com, 'This is the url returned to the callback',$GET[url]); to my php script (shown in the question). This sent me an email when I sent an image to the API for processing and thus confirmed that the API was making the call back..
The curl code in the question is good. It makes a successful request to the API.
Lastly, the curl code is for a request to the CloudConvert API, version 1. There is a version 2 but the code above is not good for that. Also, there's an earlier curl request which must be used in conjunction with the one above which I'll post later for the sake of completeness.