Issue sending post request to API | PHP - php

Question

Essentially I need to send the appropriate POST request to the following Parcel Tracking API that return the shipping data from the courier provided:
API Doc: https://www.kd100.com/docs/real-time-shipment-tracking
The problem I am having, is the response:
[code] => 104
[message] => Invalid signature
Can someone please have a look at my request below and tell me if I've made a mistake based on the doc? Assuming the Key, Secret Key and Tracking data is correct, my request should be completely valid.
My POST Request so far:
$key = 'KEY';
$secret = 'SECRET KEY';
$param = array (
'carrier_id' => 'usps',
'tracking_number' => 'TRACKING #'
);
$post_data = array();
$post_data['param'] = json_encode($param, JSON_UNESCAPED_UNICODE);
$header_data = array();
$header_data['API-Key'] = $key;
$sign = md5($post_data['param'].$key.$secret);
$header_data['signature'] = strtoupper($sign);
$url = 'https://www.kd100.com/api/v1/tracking/realtime';
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_HEADER, 0);
curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($post_data));
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, false);
curl_setopt($ch, CURLOPT_HTTPHEADER, array(
"Content-Type: application/json",
"signature: $sign",
"API-Key: $key"
));
$result = curl_exec($ch);
$data = json_decode($result, true);
echo '<br/><br/>Start：<br/><pre>';
echo print_r($data);
echo '</pre>';

Answer 1

Your signature is not in uppercase. You set $sign variable but never actually set uppercase to it. You actually set uppercase to $header_data['signature'] and then never use it.
$sign = md5($post_data['param'].$key.$secret);
$header_data['signature'] = strtoupper($sign);
Your problem should be solved if you write it like this:
$sign = strtoupper(md5(post_data['param'].$key.$secret));
Remember to clean your code. You don't actually use $header_data anywhere