php pthreads pool collect callback (attempt)

<?php

class Task extends Thread {
    public $n;
    public $result;
    public $done = false;

    public function __construct ($n) {
        $this->n = $n;
    }

    public function run () {
        usleep(mt_rand(1,10) * 100000);
        $this->result = $this->n * $this->n;
        $this->done = true;
    }
}

class MyPool {
    private $tasks = [];

    public function submit (\Task $task) {
        array_push($this->tasks, $task);
        $task->start();
    }

    public function collect ($callback) {
        while (true) {
            $all_done = true;

            foreach ($this->tasks as $i => &$task) {
                if ($task->done) {
                    call_user_func($callback, $task);
                    unset($this->tasks[$i]);
                    unset($task);

                } else {
                    $all_done = false;
                }
            }

            if ($all_done) {
                break;
            }
        }
    }

}


$items = range(0, 5);
$pool = new MyPool;

foreach ($items as $n => $v) {
    $pool->submit(new Task($n));
}

print_r($items);

$pool->collect(function (Task $task) use (&$items) {
    $items[$task->n] = $task->result;
});

print_r($items);