cancel.php

<?php

final class CancellationTokenSource
{
    private $deferred;

    public $token;

    public function cancel()
    {
        $this->deferred->fail(new TaskCancelledException);
    }

    public function __construct()
    {
        $this->deferred = new Deferred;
        $this->token = new CancellationToken($this->deferred->promise);
    }
}

final class CancellationToken
{
    private $callbacks = [];
    
    public function __construct(Promise $promise)
    {
        $callbacks = &$this->callbacks;

        $promise->onResolve(static function () use($callbacks) {
            foreach ($callbacks as $callback) {
                $callback();
            }
        });
    }
    
    public function onCancel(callable $callback)
    {
        $this->callbacks[] = $callback;
    }
}