Product advertising API PHP invalid xml response

Hi I am a beginner programmer trying to integrate the API into my mobile app so I can view items from amazon from my app. I have followed all the 6 simple steps here but the example did not work for me, as shown here.

Here are my amazon PHP codes:
aws_signed_request.php

<?php

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

    $method = "GET";
    $host = "ecs.amazonaws.".$region; // must be in small case
    $uri = "/onca/xml";


    $params["Service"]          = "AWSECommerceService";
    $params["AWSAccessKeyId"]   = $public_key;
    $params["AssociateTag"]     = $associate_tag;
    $params["Timestamp"]        = gmdate("Y-m-dTH:i:sZ");
    $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 thus
      failing the authetication 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 with SHA256 and base64-encoding.
       The 'hash_hmac' function is only available from PHP 5 >= 5.1.2.
    */
    $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 cURL doesn't work for you, then use the 'file_get_contents'
       function as given below.
    */

    if ($xml_response === False)
    {
        return False;
    }
    else
    {
        /* parse XML */
        $parsed_xml = @simplexml_load_string($xml_response);
        return ($parsed_xml === False) ? False : $parsed_xml;
    }
}
?>

amazon_api_class.php

<?php

    require_once 'aws_signed_request.php';

    class AmazonProductAPI
    {
        /**
         * Your Amazon Access Key Id
         * @access private
         * @var string
         */
        private $public_key     = "AKIAJETPMOLIUXXXXXXX";

        /**
         * Your Amazon Secret Access Key
         * @access private
         * @var string
         */
        private $private_key    = "eHpNEaUwsf+HXXXXXXXXXXQGg7Ic2w+K5Gb6rYa";

        /**
         * Your Amazon Associate Tag
         * Now required, effective from 25th Oct. 2011
         * @access private
         * @var string
         */
        private $associate_tag  = "mpXXXX-20";

        /**
         * Constants for product types
         * @access public
         * @var string
         */

        /*
            Only three categories are listed here. 
            More categories can be found here:
            http://docs.amazonwebservices.com/AWSECommerceService/latest/DG/APPNDX_SearchIndexValues.html
        */
        const MUSIC = "Music";
        const DVD   = "DVD";
        const GAMES = "VideoGames";


        /**
         * Check if the xml received from Amazon is valid
         * 
         * @param mixed $response xml response to check
         * @return bool false if the xml is invalid
         * @return mixed the xml response if it is valid
         * @return exception if we could not connect to Amazon
         */
        private function verifyXmlResponse($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.");
                }
            }
        }


        /**
         * Query Amazon with the issued parameters
         * 
         * @param array $parameters parameters to query around
         * @return simpleXmlObject xml query response
         */
        private function queryAmazon($parameters)
        {
            return aws_signed_request("com", $parameters, $this->public_key, $this->private_key, $this->associate_tag);
        }


        /**
         * Return details of products searched by various types
         * 
         * @param string $search search term
         * @param string $category search category         
         * @param string $searchType type of search
         * @return mixed simpleXML object
         */
        public function searchProducts($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);

        }


        /**
         * Return details of a product searched by UPC
         * 
         * @param int $upc_code UPC code of the product to search
         * @param string $product_type type of the product
         * @return mixed simpleXML object
         */
        public function getItemByUpc($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);

        }


        /**
         * Return details of a product searched by ASIN
         * 
         * @param int $asin_code ASIN code of the product to search
         * @return mixed simpleXML object
         */
        public function getItemByAsin($asin_code)
        {
            $parameters = array("Operation"     => "ItemLookup",
                                "ItemId"        => $asin_code,
                                "ResponseGroup" => "Medium");

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

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


        /**
         * Return details of a product searched by keyword
         * 
         * @param string $keyword keyword to search
         * @param string $product_type type of the product
         * @return mixed simpleXML object
         */
        public function getItemByKeyword($keyword, $product_type)
        {
            $parameters = array("Operation"   => "ItemSearch",
                                "Keywords"    => $keyword,
                                "SearchIndex" => $product_type);

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

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

    }

?>

Example.php

<?php

    /* Example usage of the Amazon Product Advertising API */
    include("amazon_api_class.php");

    $obj = new AmazonProductAPI();

    try
    {
        $result = $obj->searchProducts("X-Men Origins",
                                       AmazonProductAPI::DVD,
                                       "TITLE");
    }
    catch(Exception $e)
    {
        echo $e->getMessage();
    }

    print_r($result);


    echo "Sales Rank : {$result->Items->Item->SalesRank}<br>";
    echo "ASIN : {$result->Items->Item->ASIN}<br>";
    echo "<br><img src="" . $result->Items->Item->MediumImage->URL . "" /><br>";


?>

Again, this is the result of running Example.php shown here. The result of print_r($result); is invalid xml respnse.

Answer

For my case, my error was InvalidClientTokenId , The X.509 certificate or AWS access key ID provided does not exist in our records. I simply (very carefuly) created a new access and secret keys with this guide and referred to this guide to confirm my associate ID.

To find your error, simply nullify the line with throw new Exception("Invalid xml response."); and add return ($response); and run the php page. Refer to this page for the list of errors.