Add an example for MULTI/EXEC transactions using CAS.

examples/MultiExecTransactionsWithCAS.php

@@ -0,0 +1,41 @@
+<?php
+require_once 'SharedConfigurations.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
+ZADD zset 2 b
+ZADD zset 3 c
+*/
+
+function zpop($client, $zsetKey) {
+    $element = null;
+    $options = array(
+        'cas'   => true,     // Initialize with support for CAS operations
+        'watch' => $zsetKey, // 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.
+    );
+
+    $txReply = $client->multiExec($options, function($tx) 
+        use ($zsetKey, &$element) {
+        @list($element) = $tx->zrange($zsetKey, 0, 0);
+        if (isset($element)) {
+            $tx->multi();     // With CAS, MULTI *must* be explicitly invoked.
+            $tx->zrem($zsetKey, $element);
+        }
+    });
+    return $element;
+}
+
+$redis = new Predis\Client($single_server, 'dev');
+$zpopped = zpop($redis, 'zset');
+echo isset($zpopped) ? "ZPOPed $zpopped" : "Nothing to ZPOP!", "\n";
+?>