Callback can accept not only GearmanTask object, but it can recieve a variable from GearmanClient::addTask():
<?php
$client = new GearmanClient();
$client->addServer();
$client->setCreatedCallback(function(GearmanTask $task, $some_info) {
// now we have $some_info
// ...
});
$client->addTask($function_name, $workload, "some info");
?>
So, we can send to our anonymous function something like $workload, because we can't get it from GearmanTask object. It may be usefull for logging of tasks queuing.