$uri = "http://webservices.amazon.com/onca/xml?" .
       "Service=AWSECommerceService" . 
       "&Operation=ItemSearch" . 
       "&MerchantId=All" . 
       "&Condition=All" . 
       "&Availability=Available" . 
       "&Sort={$sort_by}" . 
       "&Version={$amazon_version}" . 
       "&SubscriptionId={$amazon_subscription_id}" . 
       "&AssociateTag={$amazon_associate_tag}" . 
       "&{$amazon_search_type}={$search_title}" . 
       "&SearchIndex={$search_database}" . 
       "&ResponseGroup={$information_requested}";

/**
  * This function will take an existing Amazon request and change it so that it will be usable 
  * with the new authentication.
  *
  * @param string $secret_key - your Amazon AWS secret key
  * @param string $request - your existing request URI
  * @param string $access_key - your Amazon AWS access key
  * @param string $version - (optional) the version of the service you are using
  */
function getRequest($secret_key, $request, $access_key = false, $version = '2009-03-01') {
    // Get a nice array of elements to work with
    $uri_elements = parse_url($request);
 
    // Grab our request elements
    $request = $uri_elements['query'];
 
    // Throw them into an array
    parse_str($request, $parameters);
 
    // Add the new required paramters
    $parameters['Timestamp'] = gmdate("Y-m-d\TH:i:s\Z");
    $parameters['Version'] = $version;
    if (strlen($access_key) > 0) {
        $parameters['AWSAccessKeyId'] = $access_key;
    }   
 
    // The new authentication requirements need the keys to be sorted
    ksort($parameters);
 
    // Create our new request
    foreach ($parameters as $parameter => $value) {
        // We need to be sure we properly encode the value of our parameter
        $parameter = str_replace("%7E", "~", rawurlencode($parameter));
        $value = str_replace("%7E", "~", rawurlencode($value));
        $request_array[] = $parameter . '=' . $value;
    }   
 
    // Put our & symbol at the beginning of each of our request variables and put it in a string
    $new_request = implode('&', $request_array);
 
    // Create our signature string
    $signature_string = "GET\n{$uri_elements['host']}\n{$uri_elements['path']}\n{$new_request}";
 
    // Create our signature using hash_hmac
    $signature = urlencode(base64_encode(hash_hmac('sha256', $signature_string, $secret_key, true)));
 
    // Return our new request
    return "http://{$uri_elements['host']}{$uri_elements['path']}?{$new_request}&Signature={$signature}";
}