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/Collection/Iterator/SortedSetKey.php

@@ -0,0 +1,58 @@
+<?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\Collection\Iterator;
+
+use Predis\ClientInterface;
+
+/**
+ * Abstracts the iteration of members stored in a sorted set
+ * by leveraging the ZSCAN command (Redis >= 2.8) wrapped in
+ * a fully-rewindable PHP iterator.
+ *
+ * @author Daniele Alessandri <suppakilla@gmail.com>
+ * @link http://redis.io/commands/scan
+ */
+class SortedSetKey extends CursorBasedIterator
+{
+    protected $key;
+
+    /**
+     * {@inheritdoc}
+     */
+    public function __construct(ClientInterface $client, $key, $match = null, $count = null)
+    {
+        $this->requiredCommand($client, 'ZSCAN');
+
+        parent::__construct($client, $match, $count);
+
+        $this->key = $key;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    protected function executeCommand()
+    {
+        return $this->client->zscan($this->key, $this->cursor, $this->getScanOptions());
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    protected function extractNext()
+    {
+        $element = array_shift($this->elements);
+
+        $this->position = $element[0];
+        $this->current = $element[1];
+    }
+}