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/CommunicationException.php

@@ -0,0 +1,76 @@
+<?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;
+
+use Predis\Connection\SingleConnectionInterface;
+
+/**
+ * Base exception class for network-related errors.
+ *
+ * @author Daniele Alessandri <suppakilla@gmail.com>
+ */
+abstract class CommunicationException extends PredisException
+{
+    private $connection;
+
+    /**
+     * @param SingleConnectionInterface $connection     Connection that generated the exception.
+     * @param string                    $message        Error message.
+     * @param int                       $code           Error code.
+     * @param \Exception                $innerException Inner exception for wrapping the original error.
+     */
+    public function __construct(
+        SingleConnectionInterface $connection, $message = null, $code = null, \Exception $innerException = null
+    ) {
+        parent::__construct($message, $code, $innerException);
+        $this->connection = $connection;
+    }
+
+    /**
+     * Gets the connection that generated the exception.
+     *
+     * @return SingleConnectionInterface
+     */
+    public function getConnection()
+    {
+        return $this->connection;
+    }
+
+    /**
+     * Indicates if the receiver should reset the underlying connection.
+     *
+     * @return bool
+     */
+    public function shouldResetConnection()
+    {
+        return true;
+    }
+
+    /**
+     * Offers a generic and reusable method to handle exceptions generated by
+     * a connection object.
+     *
+     * @param CommunicationException $exception Exception.
+     */
+    public static function handle(CommunicationException $exception)
+    {
+        if ($exception->shouldResetConnection()) {
+            $connection = $exception->getConnection();
+
+            if ($connection->isConnected()) {
+                $connection->disconnect();
+            }
+        }
+
+        throw $exception;
+    }
+}