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/Text/TextCommandSerializer.php

@@ -0,0 +1,47 @@
+<?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\Text;
+
+use Predis\Command\CommandInterface;
+use Predis\Protocol\CommandSerializerInterface;
+
+/**
+ * Implements a pluggable command serializer using the standard  wire protocol
+ * defined by Redis.
+ *
+ * @link http://redis.io/topics/protocol
+ * @author Daniele Alessandri <suppakilla@gmail.com>
+ */
+class TextCommandSerializer implements CommandSerializerInterface
+{
+    /**
+     * {@inheritdoc}
+     */
+    public function serialize(CommandInterface $command)
+    {
+        $commandId = $command->getId();
+        $arguments = $command->getArguments();
+
+        $cmdlen = strlen($commandId);
+        $reqlen = count($arguments) + 1;
+
+        $buffer = "*{$reqlen}\r\n\${$cmdlen}\r\n{$commandId}\r\n";
+
+        for ($i = 0, $reqlen--; $i < $reqlen; $i++) {
+            $argument = $arguments[$i];
+            $arglen = strlen($argument);
+            $buffer .= "\${$arglen}\r\n{$argument}\r\n";
+        }
+
+        return $buffer;
+    }
+}