Please can you try the code below ?
$requestBody can be an array, querystring, json or else according to your request headers and the request handler which will recieve your http request.
contents of the test-curl.php:
<?PHP
error_reporting();
ini_set('display_errors', 'On');
function makeRequest($url, $requestBody)
{
    $handle = curl_init();
    $headers = array(); //array of request headers
    //Example headers for standart browser request
    //$headers = array(
    //   'Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8');
    curl_setopt($handle, CURLOPT_URL, $url);
    curl_setopt($handle, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($handle, CURLOPT_POST, 1);
    curl_setopt($handle, CURLOPT_POSTFIELDS, $requestBody);
    curl_setopt($handle, CURLOPT_HTTPHEADER, $headers);
    curl_setopt($handle, CURLOPT_HEADER, 1); //This means include headers in response
    $result = curl_exec($handle);
    $header_size = curl_getinfo($handle, CURLINFO_HEADER_SIZE);
    $responseHeaders = substr($result, 0, $header_size);
    $responseBody = substr($result, $header_size, strlen($result) - $header_size);
    $httpCode = curl_getinfo($handle, CURLINFO_HTTP_CODE);
    if ($httpCode == 200)
        return $responseBody;
    else
        throw new Exception($responseBody);
}
$method = $_SERVER["REQUEST_METHOD"];
if ($method == "GET") {
    $request = array();
    $request["a"] = "1";
    $request["b"] = "3";
    $request["c"] = "4";
    $request["d"] = "7";
    //$request = "a=1&b=3&c=4&d=7"; //This is the same with array version for standart post request.
    $response = makeRequest("http://localhost/test-curl.php", $request);
    echo $response;
} else {
    print_r($_POST);
}
?>
If you run your test-curl.php with http://localhost/test-curl.php url it will make a post request to itself and you will see the print_r output of $_POST array. Something like below;
Array ( [a] => 1 [b] => 3 [c] => 4 [d] => 7 ) 
Hope this helps you.