Here is your code, a bit clean, with an example function showing how to extend this approach to other AWS APIs.
function aws_query($extraparams) { $private_key = ACCESS_SECRET_KEY; $method = "GET"; $host = "webservices.amazon.com"; $uri = "/onca/xml"; $params = array( "AssociateTag" => ASSOCIATE_TAG, "Service" => "AWSECommerceService", "AWSAccessKeyId" => ACCESS_KEY_ID, "Timestamp" => gmdate("Ymd\TH:i:s\Z"), "SignatureMethod" => "HmacSHA256", "SignatureVersion" => "2", "Version" => "2013-08-01" ); foreach ($extraparams as $param => $value) { $params[$param] = $value; } ksort($params); // sort the parameters // create the canonicalized query $canonicalized_query = array(); foreach ($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); // create the string to sign $string_to_sign = $method . "\n" . $host . "\n" . $uri . "\n" . $canonicalized_query; // calculate HMAC with SHA256 and base64-encoding $signature = base64_encode( hash_hmac("sha256", $string_to_sign, $private_key, True)); // encode the signature for the equest $signature = str_replace("%7E", "~", rawurlencode($signature)); // Put the signature into the parameters $params["Signature"] = $signature; uksort($params, "strnatcasecmp"); // TODO: the timestamp colons get urlencoded by http_build_query // and then need to be urldecoded to keep AWS happy. Spaces // get reencoded as %20, as the + encoding doesn't work with // AWS $query = urldecode(http_build_query($params)); $query = str_replace(' ', '%20', $query); $string_to_send = "https://" . $host . $uri . "?" . $query; return $string_to_send; } function aws_itemlookup($itemId) { return aws_query(array ( "Operation" => "ItemLookup", "IdType" => "ASIN", "ItemId" => $itemId )); }