Coinpayments create_transaction "ERROR: Invalid command!" - php

Question

I am trying to let people pay on my site with a simple API of Coinpayments (I tought it was simple). So, I found this page of the official Coinpayments website on how to create a transaction and receive money.
So, I am trying to receive the response as JSON en just echo it for now, so I can see what further steps I will take. Now, this is my code:
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL,"https://www.coinpayments.net/index.php");
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS,
"cmd=create_transaction&amount=".$amount."&currency1=USD&currency2=BTC&buyer_email=".$email."&version=1&key=b07f0fee01909b919235d58a950378b8c0e5266fa2174e007b24220a592aa92e&format=json");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
$server_output = curl_exec($ch);
curl_close ($ch);
When I try to echo the $server_output, it gives this: ERROR: Invalid command!. I have searched around, and I couldn't find anybody having the same issue as I do.
Thanks for the help! <3

Answer 1

This is what works for me:
function curl_coin_payment ($postdata = array()) {
$url = "https://www.coinpayments.net/api.php";
$payload = $postdata;
$payload['key'] = 'public_key';
$payload['version'] = 1;
$payload = http_build_query($payload, '', '&');
$api_secret = 'private_key';
$apiseal = hash_hmac('sha512', ($payload), $api_secret);
$ch = curl_init(); curl_setopt($ch, CURLOPT_URL, $url); curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, ($payload)); curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
$headers = [
"HMAC: $apiseal",
"Content-Type: application/x-www-form-urlencoded",
];
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers); $request = curl_exec ($ch); curl_close ($ch);
return $request;
}