test2.php

<?php

final class Worker
{
    private $id;
    private $handle;
    private $mutex;
    private $condition;
    private $loop;
    private $callables;

    public function __construct()
    {
        $this->mutex     = fhread_mutex_init();
        $this->condition = fhread_cond_init();
        $this->loop      = true;
        $this->callables = [];
    }

    public function __destruct()
    {
        fhread_cond_destroy(
            $this->condition
        );

        fhread_mutex_destroy(
            $this->mutex
        );
    }

    public function start()
    {
        fhread_create(
            $this,
            $this->id,
            $this->handle
        );
    }

    public function run()
    {
        do {

            fhread_mutex_lock(
                $this->mutex
            );

            if (count($this->callables) === 0) {

                fhread_cond_wait(
                    $this->condition,
                    $this->mutex
                );
            }

            $callable = array_shift(
                $this->callables
            );

            fhread_mutex_unlock(
                $this->mutex
            );

            $callable();

        } while ($this->loop);
    }

    public function assign(callable $callable)
    {
        fhread_mutex_lock(
            $this->mutex
        );

        $this->callables[] = $callable;

        fhread_mutex_unlock(
            $this->mutex
        );

        fhread_cond_signal(
            $this->condition
        );
    }

    public function join()
    {
        $error = null;

        $this->assign(function () {

            $this->loop = false;
        });

        fhread_join(
            $this->handle,
            $error
        );
    }
}

$worker = new Worker;
$worker->start();
$worker->assign(function () { echo 'Task 1.' . PHP_EOL; });
$worker->assign(function () { echo 'Task 2.' . PHP_EOL; });
$worker->assign(function () { echo 'Task 3.' . PHP_EOL; });
$worker->assign(function () { echo 'Task 4.' . PHP_EOL; });
$worker->assign(function () { echo 'Task 5.' . PHP_EOL; });
$worker->join();