Very strange!This error makes me crazy!

I am using amazon advertising api.There is a big problem i came cross.
I used this unsigned url:
http://ecs.amazonaws.com/onca/xml?Service=AWSECommerceService
&Version=2009-10-01
&Operation=ItemSearch
&SearchIndex=Apparel
&BrowseNode=1046310
&Brand=Ray-Ban
&ResponseGroup=ItemAttributes,Offers,Images,SearchBins
&MerchantId=All
then i got signed url below via Signed Requests Helper and this signed url works well.
http://ecs.amazonaws.com/onca/xml?A...=ZVgn6fDrlnslT80lXqsGDk7NEmmCpUtysiLIlMHM3OA=

But,in my php file,there is always an error said signature is not a valid value .
Here is my php code:
$AWSAccessKeyId = "AKIAJ5XYNA22LKPKD54Q";
$SecretAccessKey = "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";
$Timestamp = gmdate("Y-m-d\TH:i:s\Z");
$ResponseGroup = "ItemAttributes,Offers,Images,SearchBins";
$Node="1046310";

$String = "AWSAccessKeyId=$AWSAccessKeyId&
BrowseNode=1046310&
Brand=Ray-Ban&
MerchantId=All&
Operation=ItemSearch&
ResponseGroup=$ResponseGroup&
SearchIndex=Apparel&
Service=AWSECommerceService&
Timestamp=$Timestamp&
Version=2009-10-01";

$String = preg_replace('/[\r\n]/', '', $String);
$String = str_replace("%3D", "=", str_replace("%26", "&", rawurlencode($String)));
$Prepend = "GET\necs.amazonaws.com\n/onca/xml\n";
$PrependString = $Prepend . $String;
$Signature = rawurlencode(base64_encode(hash_hmac("sha256", $PrependString, $SecretAccessKey, True)));

$BaseUrl = "http://ecs.amazonaws.com/onca/xml?";
$SignedRequest = $BaseUrl . $String . "&Signature=" . $Signature;

$XML = simplexml_load_file($SignedRequest);

BTW,my code works well after i delete string "Brand=Ray-Ban" .
Why! i am already crazy!

 

This sounds pretty technical. Contact their support team for this one.

 

I have found the reason .

 

Can you post it.

 

Name-value pairs must be sorted from A to Z.

 

thanks ...........