在PHP中编码Amazon Flexible Payments秘密字符串的问题

这是完整的签名,所以你可以看到我添加了签名方法：

$string_to_sign = "GETn
authorize.payments-sandbox.amazon.comn
cobranded-ui/actions/start?
SignatureMethod=HmacSHA256&SignatureVersion=2&callerKey=my_key&callerReference=YourCallerReference&paymentReason=donation&pipelineName=SingleUse&returnUrl=http%3A%2F%2Fyourwebsite.com%2Freturn.html&transactionAmount=4.0";

然后我加密它如下.

$encoded_string_to_sign = URLEncode(Base64_Encode(hash_hmac("sha256",$string_to_sign,'my_secret_key')));

我这样做,但后来我从他们那里得到一个错误说：

Caller Input Exception: The following input(s) are either invalid or absent:[signatureMethod]

知道这里可能出了什么问题吗？

以下是整个代码:(变量分配上面的值)

<?php
$string_to_sign = 'GET
authorize.payments-sandbox.amazon.com/cobranded-ui/actions/startSignatureMethod=HmacSHA256&SignatureVersion=2&callerKey=AKIAJENBYSJCJX2IDWDQ&callerReference=YourCallerReference&paymentReason=donation&pipelineName=SingleUse&returnUrl=http%3A%2F%2Fproblemio.com&transactionAmount=4.0';

    $encoded_string_to_sign = URLEncode(Base64_Encode(hash_hmac("sha256",'my_secret_key')));

$amazon_request_sandbox = 'https://authorize.payments-sandbox.amazon.com/cobranded-ui/actions/start?SignatureVersion=2&returnUrl='.$return_url.'&paymentReason='.$payment_reason.'&callerReference=YourCallerReference&callerKey='.$my_access_key_id.'&transactionAmount=4.0&pipelineName=SingleUse&SignatureMethod=HmacSHA256&Signature='.$encoded_string_to_sign;

//echo $amazon_request_sandbox; - use this if you want to see the resulting request and paste it into the browser

header('Location: '.$amazon_request_sandbox);
?>

谢谢！！