Papara curl null responding

For a while solving papara payment issues. i think i did everything correctly but curl not responding me..
Here is documentation https://merchant-api.papara.com/

and here is my codes for Qr payment

$paparaID        = 123456;
$api_key         = 'XXX';
$secure_key      = 'XXX';
$environment_url = 'https://merchant-api.papara.com/qrpayments';

$payload = array(
    "amount"           => 99.99, //opsiyonel
    "referenceId"      => $paparaID,
    "orderDescription" => "Kullanıcının ödeme sayfasında göreceği açıklama",
    "notificationUrl"  => "https://www.oyuntuccari.com/bakiye/obpapara", //opsiyonel
    "redirectUrl"      => "https://www.oyuntuccari.com/bakiye/obpapara", //opsiyonel
    "currency"         => 0 //opsiyonel
);

$header = [
    "ApiKey:" . $api_key,
    "Content-Type: application/json"
];

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $environment_url);
curl_setopt($ch, CURLOPT_HTTPHEADER, $header);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($payload));
curl_setopt($ch, CURLOPT_FRESH_CONNECT, true);
curl_setopt($ch, CURLOPT_TIMEOUT, 50);

$result = curl_exec($ch);

if (curl_errno($ch)) {
    print "Error: " . curl_error($ch);
    exit();
}

curl_close($ch);

$result = json_decode($result, 1);

I hope papara developers help me on stackoverflow.