Very simple and easy to use request signer for Amazon Product Advertising API. The only method returns ready-to-send URL

AmazonSigner.php

<?php
/**
 * Amazon Product Advertising API Signer
 *
 * @author Alexey Kopytko
 * @license MIT
 *
 * Usage:
 *
 * $readyURL = AmazonSigner::signedURL([
 *     'Service'=>'AWSECommerceService',
 *     'Operation'=>'ItemLookup',
 *     'Version'=>'2011-08-01',
 *     'ItemId'=>'059035342X',
 *     'IdType'=>'ASIN',
 *     'ResponseGroup'=>'Images,ItemAttributes,Offers',
 * ]);
 */

class AmazonSigner
{
    /*
     * Product Advertising API ScratchPad:
     * http://associates-amazon.s3.amazonaws.com/scratchpad/index.html
     */
    const ACCESS_KEY_ID = 'your-key';
    const SECRET_ACCESS_KEY = 'your-secret';
    const ASSOCIATE_TAG = 'your-tag';

    const REQUEST_PROTO = 'http://';
    const REQUEST_METHOD = 'GET';
    const SERVICE_HOST = 'webservices.amazon.com';
    const SERVICE_URI = '/onca/xml';

    /**
     * @param array $request
     * @return string URL
     */
    public static function signedURL($request)
    {
        $request['AWSAccessKeyId'] = self::ACCESS_KEY_ID;
        $request['AssociateTag'] = self::ASSOCIATE_TAG;
        $request['Timestamp'] = gmdate('c');

        // Amazon wants comma-separated arrays
        $request = array_map(function ($value) {
            return is_array($value) ? implode(',', $value) : (string) $value;
        }, $request);

        // Amazon wants byte-sorted order for signatures
        ksort($request);

        $request['Signature'] = base64_encode(hash_hmac('sha256', implode("\n", [
            self::REQUEST_METHOD,
            self::SERVICE_HOST,
            self::SERVICE_URI,
            http_build_query($request)
        ]), self::SECRET_ACCESS_KEY, true));

        return implode([
            self::REQUEST_PROTO,
            self::SERVICE_HOST,
            self::SERVICE_URI,
            '?', http_build_query($request)
        ]);
    }
}