Amazon request signature does not match the signature you provided - php

Question

I am trying to get Product Reviews from Amazon but getting this error ,
The request signature we calculated does not match the signature you provided. Check your AWS Secret Access Key and signing method. Consult the service documentation for details.
Below is my code ,
define('AMAZON_URL','http://webservices.amazon.com/onca/xml?');
define('AMAZON_ACCESS_KEY',--access_key--);
define('AMAZON_SECRET_KEY',--secret_key--);
define('AMAZON_ASSOCIATE_TAG',--tag--);
$dr['model_no'] = --model_no--;
$dr['manufacturer'] = --manufacturer--;
//get amazon data
$timestamp = gmdate("Y-m-d\TH:i:s\Z");//rawurlencode(gmdate("Y-m-d\TH:i:s\Z"));
$url = "AWSAccessKeyId=".AMAZON_ACCESS_KEY;
$url .= "&AssociateTag=".AMAZON_ASSOCIATE_TAG;
$url .= "&IncludeReviewsSummary=True";
$url .= '&Keywords='.$dr['model_no'];
$url .= "&Manufacturer=".$dr['manufacturer'];
$url .= "&Operation=ItemSearch";
$url .= "&ResponseGroup=Reviews";
$url .= "&ReviewSort=HelpfulVotes";
$url .= "&SearchIndex=Electronics";
$url .= "&Service=AWSECommerceService";
$url .= "&Timestamp=".$timestamp;
$url .= "&Version=2013-05-31";
$prepend = "GET\nwebservices.amazon.com\n/onca/xml\n";
$prepend_string = $prepend . $url;
$signature = base64_encode(hash_hmac("sha256", $prepend_string, AMAZON_SECRET_KEY, True));
$signature = str_replace("+", "%2B", $signature);
$signature = str_replace("=", "%3D", $signature);
// $signature = base64_encode(hash_hmac("sha256", $prepend_string, AMAZON_SECRET_KEY, True));
$url = $url.'&Signature=' .$signature;
// $url = $url.'&Signature=' .rawurlencode($signature);
echo "$url\n";
$result = $this->data_api->call_amazon($url);
My call_amazon function is
public function call_amazon($url)
{
$url = AMAZON_URL.$url;
//initialize curl
echo "Caalling url: \n" . $url . "\n";
$ch = curl_init();
//initilize url to set which servvice to call
curl_setopt($ch, CURLOPT_URL, $url);
//to tell curl that post method is used
//tell that retrun value as string
curl_setopt($ch, CURLOPT_RETURNTRANSFER , 1);
//execute service now
$result = curl_exec($ch);
if(empty($result) || $result===FALSE){
return '<customError>
<status>400</status>
<response>false</response>
</customError>';
} else {
echo "$result \n";
//return response
return $result;
}
}
I had checked the docs and examples and signature looks ok to me. Any suggestions?