Invalid signature on amazon v2 - php

Question

I have a problem on setting signature in this part of the guide (http://amazonpaycheckoutintegrationguide.s3.amazonaws.com/amazon-pay-checkout/set-payment-info.html). Here' my code:
<?
header('Content-Type: application/json');
define("STORE_ID", "amzn1.application-oa2-client.fb120c0b541e4007aaf987a73b365a3e");
define("VENDOR_ID", "A6SFQPANHYSL0");
define("PUBLIC_KEY_ID", "AGBUUNBAKQW5OMTKHP5WZH55");
define("PRIVATE_KEY_ID", "AmazonPay_AGBUUNBAKQW5OMTKHP5WZH55.pem");
$method = 'POST';
// API Merchant Scan
$url = 'https://pay-api.amazon.eu/sandbox/v2/checkoutSessions/'.$_GET['amazonCheckoutSessionId'];
$payload = array(
'webCheckoutDetails' => array(
'checkoutResultReturnUrl'=> 'https://a.com/merchant-confirm-page'
),
'paymentDetails' => array(
'paymentIntent'=> 'AuthorizeWithCapture',
'canHandlePendingAuthorization'=>false,
'softDescriptor'=> 'Descriptor',
'chargeAmount'=> array(
'amount'=> '1',
'currencyCode'=> 'EUR'
),
),
'merchantMetadata'=> array(
'merchantReferenceId'=> 'Merchant reference ID',
'merchantStoreName'=> 'Merchant store name',
'noteToBuyer'=> 'Note to buyer',
'customInformation'=> 'Custom information'
)
);
// Convert to json string
$payload = json_encode($payload);
$requestParameters = array();
include 'amazon-pay-api-sdk-php-master/vendor/autoload.php';
$amazonpay_config = array(
'public_key_id' => PUBLIC_KEY_ID,
'private_key' => PRIVATE_KEY_ID,
'region' => 'EU',
'sandbox' => true
);
$client = new Amazon\Pay\API\Client($amazonpay_config);
// Create an array that will contain the parameters for the charge API call
$pre_signed_headers = array();
$pre_signed_headers['Accept'] = 'application/json';
$pre_signed_headers['Content-Type'] = 'application/json';
$pre_signed_headers['X-Amz-Pay-Region'] = 'eu';
$timestamp_data = date("Ymd");
$timestamp_orario = date("His");
$timestamp = $timestamp_data."T".$timestamp_orario."Z";
$signedInput = $client->createSignature($method, $url, $requestParameters, $pre_signed_headers, $payload, $timestamp);
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, 'https://pay-api.amazon.eu/sandbox/v2/checkoutSessions/'.$_GET['amazonCheckoutSessionId']);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, 'PATCH');
curl_setopt($ch, CURLOPT_POSTFIELDS, $payload);
$headers = array();
$headers[] = 'Authorization: AMZN-PAY-RSASSA-PSS PublicKeyId=AGBUUNBAKQW5OMTKHP5WZH55, SignedHeaders=accept;content-type;x-amz-pay-date;x-amz-pay-host;x-amz-pay-region, Signature= '.$signedInput;
$headers[] = 'X-Amz-Pay-Date: '.$timestamp;
$headers[] = 'Content-Type: application/json';
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
$result = curl_exec($ch);
if (curl_errno($ch)) {
echo 'Error:' . curl_error($ch);
}
curl_close($ch);
$json = json_decode($result);
print_r($json);
?>
The class for $client->createSignature is:
public function createSignature($http_request_method, $request_uri, $request_parameters, $pre_signed_headers, $request_payload, $timeStamp)
{
$rsa = $this->setupRSA();
$pre_signed_headers['X-Amz-Pay-Date'] = $timeStamp;
$pre_signed_headers['X-Amz-Pay-Host'] = $this->getHost($request_uri);
$hashedPayload = $this->hexAndHash($request_payload);
$canonicalURI = $this->getCanonicalURI($request_uri);
$canonicalQueryString = $this->createCanonicalQuery($request_parameters);
$canonicalHeader = $this->getHeaderString($pre_signed_headers);
$signedHeaders = $this->getCanonicalHeadersNames($pre_signed_headers);
$canonicalRequest = (
$http_request_method . "\n" .
$canonicalURI . "\n" .
$canonicalQueryString . "\n" .
$canonicalHeader . "\n" .
$signedHeaders . "\n" .
$hashedPayload
);
$hashedCanonicalRequest = self::AMAZON_SIGNATURE_ALGORITHM . "\n" . $this->hexAndHash($canonicalRequest);
$signature = $rsa->sign($hashedCanonicalRequest);
if ($signature === false) {
throw new \Exception('Unable to sign request, is your RSA private key valid?');
}
return base64_encode($signature);
}
The problem i received from the page i want to laod is:
[reasonCode] => InvalidRequestSignature
[message] => Unable to verify signature, signing String ...
Do you know how i can get a valid signature? I can use the one i got 2 steps before (the one that create the button for amazon pay) but i don't think it is the same.
Thank you for your time.