What do I replace in this Amazon API code to make it work?

Question

I'm trying to get an output from Amazon Product API, using code from this website.

It has 3 parts:

1. signed request
2. api class
3. query

As an Amazon assosciate, I have 3 values I need to insert to the code:

1. Associate Tag
2. Public Key (General key)
3. Private Key (Secret key)

In the API class, it's clear where to insert these values:

require_once 'aws_signed_request.php';

class AmazonProductAPI
{

    private $public_key     = "YOUR AMAZON ACCESS KEY ID";
    private $private_key    = "YOUR AMAZON SECRET KEY";

    /* 'Associate Tag' now required, effective from 25th Oct. 2011 */
    private $associate_tag  = "YOUR AMAZON ASSOCIATE TAG";

    const MUSIC = "Music";
    const DVD   = "DVD";
    const GAMES = "VideoGames";

    private function verifyXmlResponse($response)
    {
        if ($response === False)
        {
            throw new Exception("Could not connect to Amazon");
        }
        else
        {
            if (isset($response->Items->Item->ItemAttributes->Title))
            {
                return ($response);
            }
            else
            {
                throw new Exception("Invalid xml response.");
            }
        }
    }

    private function queryAmazon($parameters)
    {
        return aws_signed_request("com",
                                  $parameters,
                                  $this->public_key,
                                  $this->private_key,
                                  $this->associate_tag);
    }

    public function searchProducts($search,$category,$searchType="UPC")
    {
        $allowedTypes = array("UPC", "TITLE", "ARTIST", "KEYWORD");
        $allowedCategories = array("Music", "DVD", "VideoGames");

        switch($searchType) 
        {
            case "UPC" :
                $parameters = array("Operation"     => "ItemLookup",
                                    "ItemId"        => $search,
                                    "SearchIndex"   => $category,
                                    "IdType"        => "UPC",
                                    "ResponseGroup" => "Medium");
                            break;

            case "TITLE" :
                $parameters = array("Operation"     => "ItemSearch",
                                    "Title"         => $search,
                                    "SearchIndex"   => $category,
                                    "ResponseGroup" => "Medium");
                            break;

        }

        $xml_response = $this->queryAmazon($parameters);

        return $this->verifyXmlResponse($xml_response);

    }

    public function getItemByUpc($upc_code, $product_type)
    {
        $parameters = array("Operation"     => "ItemLookup",
                            "ItemId"        => $upc_code,
                            "SearchIndex"   => $product_type,
                            "IdType"        => "UPC",
                            "ResponseGroup" => "Medium");

        $xml_response = $this->queryAmazon($parameters);

        return $this->verifyXmlResponse($xml_response);

    }

    public function getItemByAsin($asin_code)
    {
        $parameters = array("Operation"     => "ItemLookup",
                            "ItemId"        => $asin_code,
                            "ResponseGroup" => "Medium");

        $xml_response = $this->queryAmazon($parameters);

        return $this->verifyXmlResponse($xml_response);
    }

    public function getItemByKeyword($keyword, $product_type)
    {
        $parameters = array("Operation"   => "ItemSearch",
                            "Keywords"    => $keyword,
                            "SearchIndex" => $product_type);

        $xml_response = $this->queryAmazon($parameters);

        return $this->verifyXmlResponse($xml_response);
    }

}

But this API class is initially using the Sign Request. In the Sign Request, I don't understand where to insert these 3 values:

<?php

function  aws_signed_request($region,
                             $params,
                             $public_key,
                             $private_key,
                             $associate_tag)
{

    $method = "GET";
    $host = "ecs.amazonaws.".$region;
    $uri = "/onca/xml";


    $params["Service"]          = "AWSECommerceService";
    $params["AWSAccessKeyId"]   = $public_key;
    $params["AssociateTag"]     = $associate_tag;

    $params["Timestamp"]        = gmdate("Y-m-d\TH:i:s\Z");
    $params["Version"]          = "2009-03-31";

    /* The params need to be sorted by the key, as Amazon does this at
      their end and then generates the hash of the same. If the params
      are not in order then the generated hash will be different from
      Amazon thus failing the authentication process.
    */
    ksort($params);

    $canonicalized_query = array();

    foreach ($params as $param=>$value)
    {
        $param = str_replace("%7E", "~", rawurlencode($param));
        $value = str_replace("%7E", "~", rawurlencode($value));
        $canonicalized_query[] = $param."=".$value;
    }

    $canonicalized_query = implode("&", $canonicalized_query);

    $string_to_sign = $method."\n".$host."\n".$uri."\n".
                            $canonicalized_query;

    /* calculate the signature using HMAC, SHA256 and base64-encoding */
    $signature = base64_encode(hash_hmac("sha256", 
                                  $string_to_sign, $private_key, True));

    /* encode the signature for the request */
    $signature = str_replace("%7E", "~", rawurlencode($signature));

    /* create request */
    $request = "http://".$host.$uri."?".$canonicalized_query."&Signature=".$signature;

    /* I prefer using CURL */
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL,$request);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($ch, CURLOPT_TIMEOUT, 15);
    curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);

    $xml_response = curl_exec($ch);

    if ($xml_response === False)
    {
        return False;
    }
    else
    {
        /* parse XML and return a SimpleXML object, if you would
           rather like raw xml then just return the $xml_response.
         */
        $parsed_xml = @simplexml_load_string($xml_response);
        return ($parsed_xml === False) ? False : $parsed_xml;
    }
}
?>

Answer 1

You don't need to provide them in aws_signed_request.php, the credentials are passed to the function from the AmazonProductAPI class when it calls the function aws_signed_request:

private function queryAmazon($parameters)
{
    return aws_signed_request("com",
                              $parameters,
                              $this->public_key,  // <--- here!
                              $this->private_key,
                              $this->associate_tag);
}

The function isn't actually called until that point, the require just makes the function available.