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/ResponseError.php

@@ -0,0 +1,59 @@
+<?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;
+
+/**
+ * Represents an error returned by Redis (-ERR replies) during the execution
+ * of a command on the server.
+ *
+ * @author Daniele Alessandri <suppakilla@gmail.com>
+ */
+class ResponseError implements ResponseErrorInterface
+{
+    private $message;
+
+    /**
+     * @param string $message Error message returned by Redis
+     */
+    public function __construct($message)
+    {
+        $this->message = $message;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function getMessage()
+    {
+        return $this->message;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function getErrorType()
+    {
+        list($errorType, ) = explode(' ', $this->getMessage(), 2);
+
+        return $errorType;
+    }
+
+    /**
+     * Converts the object to its string representation.
+     *
+     * @return string
+     */
+    public function __toString()
+    {
+        return $this->getMessage();
+    }
+}