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/SetIsMember.php

@@ -0,0 +1,35 @@
+<?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;
+
+/**
+ * @link http://redis.io/commands/sismember
+ * @author Daniele Alessandri <suppakilla@gmail.com>
+ */
+class SetIsMember extends PrefixableCommand
+{
+    /**
+     * {@inheritdoc}
+     */
+    public function getId()
+    {
+        return 'SISMEMBER';
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function parseResponse($data)
+    {
+        return (bool) $data;
+    }
+}