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/OptionInterface.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\Option;
+
+/**
+ * Interface that defines a client option.
+ *
+ * @author Daniele Alessandri <suppakilla@gmail.com>
+ */
+interface OptionInterface
+{
+    /**
+     * Filters (and optionally converts) the passed value.
+     *
+     * @param  ClientOptionsInterface $options Options container.
+     * @param  mixed                  $value   Input value.
+     * @return mixed
+     */
+    public function filter(ClientOptionsInterface $options, $value);
+
+    /**
+     * Returns a default value for the option.
+     *
+     * @param  ClientOptionsInterface $options Options container.
+     * @return mixed
+     */
+    public function getDefault(ClientOptionsInterface $options);
+
+    /**
+     * Filters a value and, if no value is specified, returns the default one
+     * defined by the option.
+     *
+     * @param  ClientOptionsInterface $options Options container.
+     * @param  mixed                  $value   Input value.
+     * @return mixed
+     */
+    public function __invoke(ClientOptionsInterface $options, $value);
+}