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/Command/Processor/KeyPrefixProcessor.php

@@ -0,0 +1,72 @@
+<?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\Command\Processor;
+
+use Predis\Command\CommandInterface;
+use Predis\Command\PrefixableCommandInterface;
+
+/**
+ * Command processor that is used to prefix the keys contained in the arguments
+ * of a Redis command.
+ *
+ * @author Daniele Alessandri <suppakilla@gmail.com>
+ */
+class KeyPrefixProcessor implements CommandProcessorInterface
+{
+    private $prefix;
+
+    /**
+     * @param string $prefix Prefix for the keys.
+     */
+    public function __construct($prefix)
+    {
+        $this->setPrefix($prefix);
+    }
+
+    /**
+     * Sets a prefix that is applied to all the keys.
+     *
+     * @param string $prefix Prefix for the keys.
+     */
+    public function setPrefix($prefix)
+    {
+        $this->prefix = $prefix;
+    }
+
+    /**
+     * Gets the current prefix.
+     *
+     * @return string
+     */
+    public function getPrefix()
+    {
+        return $this->prefix;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function process(CommandInterface $command)
+    {
+        if ($command instanceof PrefixableCommandInterface && $command->getArguments()) {
+            $command->prefixKeys($this->prefix);
+        }
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function __toString()
+    {
+        return $this->getPrefix();
+    }
+}