无法从amazon mws产品api检索结果

使用下面的代码，我可以得到的唯一结果是URL。当我复制粘贴到浏览器中，我得到:

<Message>

我们计算的请求签名与您提供的签名不匹配。检查您的AWS秘密访问密钥和签名方法。有关详细信息，请参阅服务文档。

使用的代码;

<?php
date_default_timezone_set('America/New_York');
define('AWS_ACCESS_KEY_ID', '***');
define('AWS_SECRET_ACCESS_KEY', '***');
define('APPLICATION_NAME', 'test');
define('APPLICATION_VERSION', '1.0');
define ('MERCHANT_ID', '***');
define ('MARKETPLACE_ID', '***');
$base_url = "https://mws.amazonservices.com/Products/2011-10-01";
$method = "GET";
$host = "mws.amazonservices.com";
$uri = "/Products/2011-10-01";
function amazon_xml($searchTerm) {
$params = array(
    'AWSAccessKeyId' => AWS_ACCESS_KEY_ID,
    'Action' => "GetCompetitivePricingForASIN",
    'SellerId' => MERCHANT_ID,
    'SignatureMethod' => "HmacSHA256",
    'SignatureVersion' => "2",
    'Timestamp'=> gmdate("Y-m-d'TH:i:s.''0''0''0''Z", time()),
    'Version'=> "2011-10-01",
    'MarketplaceId' => MARKETPLACE_ID,
    'Query' => $searchTerm,
    'QueryContextId' => "Automotive");
// Sort the URL parameters
$url_parts = array();
foreach(array_keys($params) as $key)
$url_parts[] = $key . "=" . str_replace('%7E', '~', rawurlencode($params[$key]));
sort($url_parts);
// Construct the string to sign
$url_string = implode("&", $url_parts);
$string_to_sign = "GET'nmws.amazonservices.com'n/Products/2011-10-01'n" . $url_string;
// Sign the request
$signature = hash_hmac("sha256", $string_to_sign, AWS_SECRET_ACCESS_KEY, TRUE);
// Base64 encode the signature and make it URL safe
$signature = urlencode(base64_encode($signature));
$url = "https://mws.amazonservices.com/Products/2011-10-01" . '?' . $url_string .           "&Signature=" . $signature;

$ch = curl_init();
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, false);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_URL,$url);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
$response = curl_exec($ch);
echo $url;
// return $xml if you still want to parse the data elsewhere
$xml = simplexml_load_string($response);
// return this if you just want the raw XML string
return $xml->asXML();
echo $xml->GetMatchingProductResult->Product->Identifiers->MarketplaceASIN->ASIN;
}
$searchTerm = "B00FNNLBK2";
amazon_xml($searchTerm);

//foreach ($xml->GetMatchingProductResult->Product as $product) {
// do something for each <Product>, such as output the ASIN...
//}

echo $xml;
echo $searchTerm;
echo $xml->GetMatchingProductResult->Product->Identifiers->MarketplaceASIN->ASIN;
echo $url;
?>

我确实多次验证了sign in凭证是正确的，没有抽象的前导或尾随空格。