One shall note, that callback function MUST either return a valid Gearman status code, or return nothing (do not return).

I.e. these are valid complete callbacks:

<?php
function goodCallbackOne(GearmanTask $task)
{
    print_r($task);
}
?>

<?php
function goodCallbackTwo(GearmanTask $task)
{
    print_r($task);
    return GEARMAN_SUCCESS;
}
?>

While following is NOT, unless you want your client code to fail with Gearman error 'german wait:no active file descriptors':

<?php
function badCallbackTwo(GearmanTask $task)
{
    print_r($task);
    return true;
}
?>