Rename examples and update some comments.

[ci skip]

examples/transaction_using_cas.php

@@ -0,0 +1,52 @@
+<?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.
+ */
+
+require __DIR__.'/shared.php';
+
+// This is an implementation of an atomic client-side ZPOP using the support for
+// check-and-set (CAS) operations with MULTI/EXEC transactions, as described in
+// "WATCH explained" from http://redis.io/topics/transactions
+//
+// First, populate your database with a tiny sample data set:
+//
+// ./redis-cli
+// SELECT 15
+// ZADD zset 1 a 2 b 3 c
+//
+// Then execute this script four times and see its output.
+//
+
+function zpop($client, $key)
+{
+    $element = null;
+    $options = array(
+        'cas'   => true,    // Initialize with support for CAS operations
+        'watch' => $key,    // Key that needs to be WATCHed to detect changes
+        'retry' => 3,       // Number of retries on aborted transactions, after
+                            // which the client bails out with an exception.
+    );
+
+    $client->transaction($options, function ($tx) use ($key, &$element) {
+        @list($element) = $tx->zrange($key, 0, 0);
+
+        if (isset($element)) {
+            $tx->multi();   // With CAS, MULTI *must* be explicitly invoked.
+            $tx->zrem($key, $element);
+        }
+    });
+
+    return $element;
+}
+
+$client = new Predis\Client($single_server);
+$zpopped = zpop($client, 'zset');
+
+echo isset($zpopped) ? "ZPOPed $zpopped" : "Nothing to ZPOP!", PHP_EOL;