$_POST super variable dump only contains an empty array

I am trying to send $_POST data to my server using cURL, and I am following the advice on this website https://www.delftstack.com/howto/php/php-post-request/

however when I dump the contents nothing is being saved to the array.

my code is as follows

test-api-key.php

<pre>
<?php
$data = [
    "firstName" => "Rob",
    "lastName" => "Test",
];
function submit($data) {
    $curl = curl_init();        
    curl_setopt($curl, CURLOPT_URL, "http://localhost/testserver/request.php");
    curl_setopt($curl, CURLOPT_POST, true);
    curl_setopt($curl, CURLOPT_POSTFIELDS, http_build_query($data));
    curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
    $response = curl_exec($curl);
    echo $response;
}


submit($data);

echo "var_dump". "<br>";
var_dump($_POST);
printf("$_POST");
?>
</pre>

and the receiving file, request.php, is:

<?php
echo "test-echo <br>";
echo "name: ". $_POST['firstName']. "<br>";
echo "name: ". $_POST['lastName']. "<br>";
echo $_POST[$data];
printf("$_POST");
?>

The output from both files when the test-api-key.php is loaded, is:

test-api-key.php:
test-echo 
name: Rob
name: Test
Array
var_dump
array(0) {
}
Array

and request.php output is:

test-echo
name:
name:
Array

As can be seen with the dump, nothing is being stored within the $_POST variable. Both the test-api-key.php and the request.php are within the same localhost directory, however when I implement it, with live data, it will be being sent from one server to another. I understand I will need to include a header array variable when using the live data, and I have the details for that from the live api server.

I have only included both var_dump() and printf() in order to try and get as much information about what is being held within the $_POST variable as I can for debugging.

I'm trying to understand how this all works, before implementing it with live data, hence why I am just setting up a few test page with my localhost.

source https://stackoverflow.com/questions/72671609/post-super-variable-dump-only-contains-an-empty-array