调用Amazon Route53 API PHP时发生MissingAuthenticationToken错误

我试图调用Amazon route53的'ChangeResourceRecordSetsRequest' API，但每次都会得到"缺少身份验证错误"。我不确定我在这里做错了什么。有人能看一下吗？以下是我的代码：

    $AWSAccessKeyId = "##########";
    $SecretAccessKey = "#################";
    $Timestamp = gmdate('D, d M Y H:i:s') . ' GMT';
    $Signature = base64_encode(hash_hmac("sha256",$Timestamp,$SecretAccessKey));
    $AuthenticationToken = "AWS3-HTTPS AWSAccessKeyId=".$AWSAccessKeyId.", Algorithm=HmacSHA256,Signature=".$Signature;
       $headers = array(
          'X-Amzn-Authorization: $AuthenticationToken',
          'x-amz-date: $Timestamp',
          'Content-Type: text/xml;charset=utf-8',
        );
    $endpoint = 'https://route53.amazonaws.com/2012-12-12/hostedzone/######/rrset';
    $session  = curl_init($endpoint);                       // create a curl session
    curl_setopt($session, CURLOPT_POST, true);              // POST request type
    curl_setopt($session, CURLOPT_POSTFIELDS, $xmlRequest); // set the body of the POST
    curl_setopt($session, CURLOPT_RETURNTRANSFER, true);    // return values as a string - not to std out
    curl_setopt($session, CURLOPT_HTTPHEADER, $headers);
    $responseXML = curl_exec($session);