PHProutines

challenge.php

<?php

// https://gist.github.com/elimisteve/4442820
// challenge accepted :)

require __DIR__ . '/PHProutines.php';

function intDoubler($ch, $n) {
    $ch->write($n * 2);
}

$runner = new Runner;
$ch = new Chan;
$answer = new Chan;

$runner->go('intDoubler', $ch, 10);
$runner->go('intDoubler', $ch, 20);
$runner->go(function($a, $b) use ($ch) {
    $ch->write($a + $b);
}, 30, 40);

$runner->go(function() use ($ch, $answer) {
    list($x, $y, $z) = [$ch->read(), $ch->read(), $ch->read()];
    $answer->write(sprintf('%d + %d + %d = %d', $x, $y, $z, $x + $y + $z));
});

printf("%s\n", $answer->read());

PHProutines.php

<?php

class Runner {

    public function __construct() {
        pcntl_signal(SIGCHLD, SIG_IGN);
    }

    public function go() {
        if (pcntl_fork()) return;
        $args = func_get_args();
        $func = array_shift($args);
        call_user_func_array($func, $args);
        die;
    }

}

class Chan {

    protected $size;
    protected $in;
    protected $out;

    public function __construct($size = 8192) {
        $this->size = $size;
        list($this->in, $this->out) = stream_socket_pair(
            STREAM_PF_UNIX,
            STREAM_SOCK_DGRAM,
            STREAM_IPPROTO_IP
        );
    }

    public function read() {
        return fread($this->in, $this->size);
    }

    public function write($data) {
        return fwrite($this->out, $data);
    }

}

Impressive! Looks like stream_socket_pair is built into PHP?