Q:

Amazon Web Services signature error

<?php
error_reporting(E_ALL); 
ini_set("display_errors", 1); 
$AWS_ACCESS_KEY_ID = "KEY";
$AWS_SECRET_ACCESS_KEY = "ACCESS KEY";

$base_url = "http://webservices.amazon.com/onca/xml?";
$url_params = array('Operation'=>"ItemSearch",'Service'=>"AWSECommerceService",
 'AWSAccessKeyId'=>$AWS_ACCESS_KEY_ID,'AssociateTag'=>"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
$string_to_sign = "GET\webservices.amazon.com\n/onca/xml?\n".implode("&",$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);

$url_string = implode("&",$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; 
    ?>

this returns an signature error;
why?
this is the output, keys and tag are replaced for privacy
http://webservices.amazon.com/onca/xml?AWSAccessKeyId=KEY&AssociateTag=ASSIOCATE TAG&Availability=Available&Condition=All&ItemPage=1&Keywords=Amazon&Operation=ItemSearch&ResponseGroup=Images,ItemAttributes,EditorialReview&Service=AWSECommerceService&Timestamp=2012-05-27T09:35:43.000Z&Version=2011-08-01&Signature=KEVlbW6G9ygvHheTf5m0ymguE64LEaYGDtQZQe0bCLQ%3D

A:

Not sure if you still need help with this, but it's most likely failing due not providing a valid associate tag in your call.
'AssociateTag'=>"associateTag"

So you need to change this value to:
'AssociateTag'=>"(my-assigned-associate-tag)"

You MUST use the associate tag given to you by Amazon. I think associate tags usually end with the number '20', but I can't verify that. If you don't know your associate tag, login to your affiliate account here:  Amazon affiliate page
...and it will be the 'Signed in as/Tracking ID' value in the upper left hand corner of the page.

