Dirty trick to get a transaction instance out of a cluster of connections by using a particular key tag.

predis_0_6_x.php

<?php

// Using Predis 0.6

require 'Predis.php';

$clusterClient = new Predis\Client(array(/* array of servers */));

// We use a fake command (e.g. GET) to obtain the actual connection out of a cluster 
// that is responsible for our particular key tag (or a key, but *only* that one key).
$fakeCommand = $clusterClient->createCommand('get', array("{ONLY_THE_KEY_TAG}"));
$singleConnection = $clusterClient->getConnection()->getConnection($fakeCommand);

// Unfortunately, the only way to create a new client instance for a single 
// connection that participates to a cluster is by using a method the works 
// only with connection aliases.
$alias = $singleConnection->getParameters()->alias;
if (!isset($alias)) {
    // It's just a dirty trick after all...
    throw new Exception("Whooops");
}

$transaction = $clusterClient->getClientFor($alias)->multiExec();

/* do stuff with $transaction */

predis_0_7_x.php

<?php

// Using Predis 0.7

require 'predis_autoloader.php';

$clusterClient = new Predis\Client(array(/* array of servers */));

// We use a fake command (e.g. GET) to obtain the actual connection out of a cluster 
// that is responsible for our particular key tag (or a key, but *only* that one key).
$fakeCommand = $clusterClient->createCommand('get', array("{ONLY_THE_KEY_TAG}"));
$singleConnection = $clusterClient->getConnection()->getConnection($fakeCommand);

// With 0.7.x the constructor of Predis\Client can also accept instances 
// of a connection class, so it's much more clean like this.
$singleClient = new Predis\Client($singleConnection, $clusterClient->getOptions());
$transaction = $singleClient->multiExec();

/* do stuff with $transaction */

predis_0_7_x_880d2d2.php

<?php

// Using Predis 0.7 (After Git commit 880d2d2)

require 'predis_autoloader.php';

$clusterClient = new Predis\Client(array(/* array of servers */));

// After Git commit 880d2d2, the default cluster connection class gives the ability
// to get a connection out of a cluster using a key (compatible with key tagging).
$singleConnection = $clusterClient->getConnection()->getConnectionByKey("FULL_KEY_WITH_KEYTAG");

// With 0.7.x the constructor of Predis\Client can also accept instances 
// of a connection class, so it's much more clean like this.
$singleClient = new Predis\Client($singleConnection, $clusterClient->getOptions());
$transaction = $singleClient->multiExec();

/* do stuff with $transaction */