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/Option/ClientReplication.php

@@ -0,0 +1,78 @@
+<?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\Option;
+
+use Predis\Connection\MasterSlaveReplication;
+use Predis\Connection\ReplicationConnectionInterface;
+
+/**
+ * Option class that returns a replication connection be used by a client.
+ *
+ * @author Daniele Alessandri <suppakilla@gmail.com>
+ */
+class ClientReplication extends AbstractOption
+{
+    /**
+     * Checks if the specified value is a valid instance of ReplicationConnectionInterface.
+     *
+     * @param  ReplicationConnectionInterface $connection Instance of a replication connection.
+     * @return ReplicationConnectionInterface
+     */
+    protected function checkInstance($connection)
+    {
+        if (!$connection instanceof ReplicationConnectionInterface) {
+            throw new \InvalidArgumentException('Instance of Predis\Connection\ReplicationConnectionInterface expected');
+        }
+
+        return $connection;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function filter(ClientOptionsInterface $options, $value)
+    {
+        if (is_callable($value)) {
+            $connection = call_user_func($value, $options, $this);
+
+            if (!$connection instanceof ReplicationConnectionInterface) {
+                throw new \InvalidArgumentException('Instance of Predis\Connection\ReplicationConnectionInterface expected');
+            }
+
+            return $connection;
+        }
+
+        if (is_string($value)) {
+            if (!class_exists($value)) {
+                throw new \InvalidArgumentException("Class $value does not exist");
+            }
+
+            if (!($connection = new $value()) instanceof ReplicationConnectionInterface) {
+                throw new \InvalidArgumentException('Instance of Predis\Connection\ReplicationConnectionInterface expected');
+            }
+
+            return $connection;
+        }
+
+        if ($value == true) {
+            return $this->getDefault($options);
+        }
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function getDefault(ClientOptionsInterface $options)
+    {
+        return new MasterSlaveReplication();
+    }
+}