NOTE.
The register of the callback functions needs BEFORE add a task.
This example NOT working correctly:
<?php
# add tasks
$task= $gmc->addTask("reverse", "foo", $data);
# register some callbacks
$gmc->setCreatedCallback("reverse_created");
$gmc->setDataCallback("reverse_data");
$gmc->setStatusCallback("reverse_status");
$gmc->setCompleteCallback("reverse_complete");
$gmc->setFailCallback("reverse_fail");
# run the task
$gmc->runTasks();
?>