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/Command/ZSetUnionStore.php

@@ -0,0 +1,93 @@
+<?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\Command;
+
+/**
+ * @link http://redis.io/commands/zunionstore
+ * @author Daniele Alessandri <suppakilla@gmail.com>
+ */
+class ZSetUnionStore extends PrefixableCommand
+{
+    /**
+     * {@inheritdoc}
+     */
+    public function getId()
+    {
+        return 'ZUNIONSTORE';
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    protected function filterArguments(Array $arguments)
+    {
+        $options = array();
+        $argc = count($arguments);
+
+        if ($argc > 2 && is_array($arguments[$argc - 1])) {
+            $options = $this->prepareOptions(array_pop($arguments));
+        }
+
+        if (is_array($arguments[1])) {
+            $arguments = array_merge(
+                array($arguments[0], count($arguments[1])),
+                $arguments[1]
+            );
+        }
+
+        return array_merge($arguments, $options);
+    }
+
+    /**
+     * Returns a list of options and modifiers compatible with Redis.
+     *
+     * @param  array $options List of options.
+     * @return array
+     */
+    private function prepareOptions($options)
+    {
+        $opts = array_change_key_case($options, CASE_UPPER);
+        $finalizedOpts = array();
+
+        if (isset($opts['WEIGHTS']) && is_array($opts['WEIGHTS'])) {
+            $finalizedOpts[] = 'WEIGHTS';
+
+            foreach ($opts['WEIGHTS'] as $weight) {
+                $finalizedOpts[] = $weight;
+            }
+        }
+
+        if (isset($opts['AGGREGATE'])) {
+            $finalizedOpts[] = 'AGGREGATE';
+            $finalizedOpts[] = $opts['AGGREGATE'];
+        }
+
+        return $finalizedOpts;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function prefixKeys($prefix)
+    {
+        if ($arguments = $this->getArguments()) {
+            $arguments[0] = "$prefix{$arguments[0]}";
+            $length = ((int) $arguments[1]) + 2;
+
+            for ($i = 2; $i < $length; $i++) {
+                $arguments[$i] = "$prefix{$arguments[$i]}";
+            }
+
+            $this->setRawArguments($arguments);
+        }
+    }
+}