CURLOPT_POST Fields Empty - php

Question

I'm using the following code to POST. However, the contents of $_POST are null. What am I missing here?
$orderData = "This is an order";
$json = json_encode($orderData);
curl_setopt_array($curl, array(
CURLOPT_URL => "http://localhost:9999/printpost.php",
CURLOPT_CAINFO => "C:\MAMP\conf\php7.0.0\cacert.pem",
CURLOPT_CUSTOMREQUEST => "POST",
CURLOPT_HTTPHEADER => array(
"authorization: ".$oauthToken,
"content-type: application/json",
"programid: ".$programId
),
CURLOPT_POSTFIELDS => $json
));
$response = curl_exec($curl);
$err = curl_error($curl);
curl_close($curl);
if ($err) {
echo "cURL Error #:" . $err;
} else {
echo $response;
}

Answer 1

The $_POST superglobal is only used for Content-Type: application/x-www-form-urlencoded; i.e. key/value pairs formatted like foo=1&bar=2. You're posting JSON. Consume it like this on the server side instead of using the superglobal:
$json = file_get_contents('php://input');
Additionally, I recommend the following changes in the cURL client code you posted:
Replace
CURLOPT_CUSTOMREQUEST => "POST",
with
CURLOPT_POST => 1,
And add
CURLOPT_RETURNTRANSFER => 1,

Answer 2

The POST data should not be encoded as JSON, it should be a URL-encoded string in the form name=value&name=value&.... Furthermore, even if JSON were processed, you didn't specify a name for your parameter, so what were you expecting the key in $_POST to be?
If you provide an associative array as CURLOPT_POSTFIELDS, it will automatically encode it for you.
'CURLOPT_POSTFIELDS' => array('orderData' => $orderData)
Then in the server script you can use $_POST['orderData']