Parallel task worker with kill signaling in PHP

<?php
/*
* Task worker - design 2
* Adds pub-sub flow to receive and respond to kill signal
* @author Ian Barber <ian(dot)barber(at)gmail(dot)com>
*/

$context = new ZMQContext();

// Socket to receive messages on
$receiver = new ZMQSocket($context, ZMQ::SOCKET_PULL);
$receiver->connect("tcp://localhost:5557");

// Socket to send messages to
$sender = new ZMQSocket($context, ZMQ::SOCKET_PUSH);
$sender->connect("tcp://localhost:5558");

// Socket for control input
$controller = new ZMQSocket($context, ZMQ::SOCKET_SUB);
$controller->connect("tcp://localhost:5559");
$controller->setSockOpt(ZMQ::SOCKOPT_SUBSCRIBE, "");

// Process messages from receiver and controller
$poll = new ZMQPoll();
$poll->add($receiver, ZMQ::POLL_IN);
$poll->add($controller, ZMQ::POLL_IN);
$readable = $writeable = array();

// Process messages from both sockets
while (true) {
$events = $poll->poll($readable, $writeable);
if ($events > 0) {
foreach ($readable as $socket) {
if ($socket === $receiver) {
$message = $socket->recv();
// Simple progress indicator for the viewer
echo $message, PHP_EOL;

// Do the work
usleep($message * 1000);

// Send results to sink
$sender->send("");
}
// Any waiting controller command acts as 'KILL'
else if ($socket === $controller) {
exit();
}
}
}
}