repro.php

<?php

class Wrapper {
    public $worker;
    public function __construct() {
        $this->worker = new Worker;
        $this->worker->start();
    }

    public function stack(Collectable $work) {
        $this->worker->stack($work);
    }
}

class Work extends Collectable {
    public $wrapper;
    public function __construct(Wrapper $wrapper) {
        $this->wrapper = $wrapper;
    }
    public function stack() {
        $this->wrapper->stack($this);
    }
    public function run() {
        echo "Foo\n";
    }
}

$wrapper = new Wrapper;
$work = new Work($wrapper);
$work->stack();