SignatureDoesNotMatch Error using Amazon Web Services - php

Question

The code below results in an SignatureDoesNotMatch error and haven't had much luck on their forums or with support - any suggestions as to what is wrong with the below code?
<?php
error_reporting(E_ALL);
ini_set("display_errors", 1);
$AWS_ACCESS_KEY_ID = "";
$AWS_SECRET_ACCESS_KEY = "";
$base_url = "http://ecs.amazonaws.com/onca/xml?";
$url_params = array('Operation'=>"ItemSearch",'Service'=>"AWSECommerceService",
'AWSAccessKeyId'=>$AWS_ACCESS_KEY_ID,'AssociateTag'=>"",
'Version'=>"2011-08-01",'Availability'=>"Available",'Condition'=>"All",
'ItemPage'=>"1",'ResponseGroup'=>"Images,ItemAttributes,EditorialReview",
'Keywords'=>"Amazon");
// Add the Timestamp
$url_params['Timestamp'] = gmdate("Y-m-d\TH:i:s.\\0\\0\\0\\Z", time());
// Sort the URL parameters
$url_parts = array();
foreach(array_keys($url_params) as $key)
$url_parts[] = $key."=".$url_params[$key];
sort($url_parts);
// Construct the string to sign
$string_to_sign = "GET\necs.amazonaws.com\n/onca/xml\n".implode("&",$url_parts);
$string_to_sign = str_replace('+','%20',$string_to_sign);
$string_to_sign = str_replace(':','%3A',$string_to_sign);
$string_to_sign = str_replace(';',urlencode(';'),$string_to_sign);
// Sign the request
$signature = hash_hmac("sha256",$string_to_sign,$AWS_SECRET_ACCESS_KEY,TRUE);
// Base64 encode the signature and make it URL safe
$signature = base64_encode($signature);
$signature = str_replace('+','%2B',$signature);
$signature = str_replace('=','%3D',$signature);
$signature = str_replace(';',urlencode(';'),$signature);
$url_string = implode("&",$url_parts);
$url = $base_url.$url_string."&Signature=".$signature;
print $url;
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL,$url);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_TIMEOUT, 15);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
$xml_response = curl_exec($ch);
echo $xml_response;
?>

Answer 1

I was getting similar error message:
["Code"]=> string(21) "SignatureDoesNotMatch"
["Message"]=> string(100) "Signature expired: 20130528T162908Z is now earlier than 20130528T162916Z (20130528T163416Z - 5 min.)"
After checking my server clock, I noticed that it time was not right, and after syncing my server clock sudo ntpdate ntp.ubuntu.com I was able to send emails with no problem, I hope this help.