Multithreaded relay in PHP

<?php
/*
* Multithreaded relay. Actually using processes due a lack
* of PHP threads.
* @author Ian Barber <ian(dot)barber(at)gmail(dot)com>
*/

function step1()
{
$context = new ZMQContext();
// Signal downstream to step 2
$sender = new ZMQSocket($context, ZMQ::SOCKET_PAIR);
$sender->connect("ipc://step2.ipc");
$sender->send("");
}

function step2()
{
$pid = pcntl_fork();
if ($pid == 0) {
step1();
exit();
}

$context = new ZMQContext();
// Bind to ipc: endpoint, then start upstream thread
$receiver = new ZMQSocket($context, ZMQ::SOCKET_PAIR);
$receiver->bind("ipc://step2.ipc");

// Wait for signal
$receiver->recv();

// Signal downstream to step 3
$sender = new ZMQSocket($context, ZMQ::SOCKET_PAIR);
$sender->connect("ipc://step3.ipc");
$sender->send("");
}

// Start upstream thread then bind to icp: endpoint
$pid = pcntl_fork();
if ($pid == 0) {
step2();
exit();
}

$context = new ZMQContext();
$receiver = new ZMQSocket($context, ZMQ::SOCKET_PAIR);
$receiver->bind("ipc://step3.ipc");

// Wait for signal
$receiver->recv();

echo "Test succesful!", PHP_EOL;