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.

Contrary to the documentation, this callback accepts an instance of \GearmanTask, not \GearmanClient.