As of PHP 5.3.0, you will get a warning saying that "call-time pass-by-reference" is deprecated when you use & in $client->addTask(..., ..., &$results, ...);. And as of PHP 5.4.0, call-time pass-by-reference was removed, so using it will raise a fatal error.
So that means that when you call addTask with a context parameter as in the example above like this:
<?php
$results = array();
$client->addTask("reverse", "Hello World!", &$results, "t1");
?>
You get this "call-time pass-by-reference" warning (or error). This can be avoided and still result in functional code by changing the context variable to be an object so that it is passed by reference like this:
<?php
$results = new \stdClass();
$client->addTask("reverse", "Hello World!", $results, "t1");
?>
Then for completeness, change the complete handler to expect a reference:
<?php
function reverse_complete($task, &$results) { ... }
?>
Then inside the complete handler, you can use the $results object to save your results to be accessible outside the complete handler.