class AmazonAPI{
private static $instance;

private $public_key;
private $secret_key;
private $associate_tag;//This is only for Amazon affiliates

private $wsdl_url='http://webservices.amazon.com/AWSECommerceService/AWSECommerceService.wsdl';
private $webservices_url='https://webservices.amazon.com/onca/soap?Service=AWSECommerceService';

public function __construct($publicKey, $secretKey, $associateTag){
    if(!empty($publicKey)){
        $this->public_key = $publicKey;
    }

    if(!empty($secretKey)){
        $this->secret_key = $secretKey;
    }

    if(!empty($associateTag)){
        $this->associate_tag = $associateTag;
    }
    }

    //Send a request to amazon
    public function sendRequest($request_params){
    $params['Request']=$request_params;
    $operation=$request_params['Operation'];

    if(isset($this->associate_tag)){
        $params['AssociateTag']=$this->associate_tag;
    }

    $soapy=new SoapClient(
        $this->wsdl_url,
        array('exceptions'=>1)
    );

    $soapy->__setLocation($this->webservices_url);

    $current_timestamp=gmdate("Y-m-dTH:i:sZ");
    $req_sig=$this->createSignature($operation,$current_timestamp);

    $headers_array=array(
        new SoapHeader(
            'http://security.amazonaws.com/doc/2007-01-01/',
            'AWSAccessKeyId',
            $this->public_key
        ),
        new SoapHeader(
            'http://security.amazonaws.com/doc/2007-01-01/',
            'Timestamp',
            $current_timestamp
        ),
        new SoapHeader(
            'http://security.amazonaws.com/doc/2007-01-01/',
            'Signature',
            $req_sig
        )
    );
    $soapy->__setSoapHeaders($headers_array);

    return $soapy->__soapCall($operation,array($params));
    }


//Create signature for request
protected function createSignature($operation,$timestamp){
    $the_string=$operation.$timestamp;
    return base64_encode(hash_hmac("sha256",$the_string,$this->secret_key,true));
}
}

<?php
   require('../AmazonApi.php');

  //Create API access object
  $public_key = 'PUBLIC-KEY';
  $secret_key = 'SECRET-KEY';
  $associate_tag = 'TAG';
  $amazon_api = new AmazonAPI($public_key, $secret_key, $associate_tag);



    $collection = Mage::getModel('catalog/product')
->getCollection()
->addStoreFilter(Mage_Core_Model_App::ADMIN_STORE_ID) // default store
->addAttributeToSelect('sku');
$asin = substr($sku,0,10);



 //Array of request parameters
 $params_array = array(
'Operation' => 'ItemLookup',
'IdType' => 'ASIN',
'ItemId' => $asin ,
'ResponseGroup' => 'Tracks'
 );

// returns a list of items for the search query 'Slow Magic'
$response = $amazon_api->sendRequest($params_array);

echo '<pre>';
print_r($response);
echo '</pre>';