InvalidateCacheWorkflow.php

<?php

namespace App\Workflows\InvalidateCache;

use Workflow\ActivityStub;
use Workflow\Workflow;
use Workflow\WorkflowStub;

class InvalidateCacheWorkflow extends Workflow
{
    public function execute($url)
    {
        $oldDate = yield ActivityStub::make(CheckImageDateActivity::class, $url);

        while (true) {
            yield ActivityStub::make(InvalidateCacheActivity::class, $url);

            for ($i = 0; $i < 3; ++$i) { 
                yield WorkflowStub::timer(30);

                $newDate = yield ActivityStub::make(CheckImageDateActivity::class, $url);

                if ($oldDate !== $newDate) return;    
            }
        }
    }
}