Retwis ported to modern Redis data types.

The example (and related documentation at redis.io) was not updated for
5 years. Redis had no sorted sets and hashes when the original code was
written (!). An update was really needed.

We also use a modern PHP client now: Predis. A copy is shipped within
this repository to make life easier to newcomers trying Redis for the
first time via this example.

Predis/Protocol/CommandSerializerInterface.php

@@ -0,0 +1,30 @@
+<?php
+
+/*
+ * This file is part of the Predis package.
+ *
+ * (c) Daniele Alessandri <suppakilla@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Predis\Protocol;
+
+use Predis\Command\CommandInterface;
+
+/**
+ * Interface that defines a custom serializer for Redis commands.
+ *
+ * @author Daniele Alessandri <suppakilla@gmail.com>
+ */
+interface CommandSerializerInterface
+{
+    /**
+     * Serializes a Redis command.
+     *
+     * @param  CommandInterface $command Redis command.
+     * @return string
+     */
+    public function serialize(CommandInterface $command);
+}