5s, times match
// Second visit -> instant, times don't match
// … T+14 -> instant, times don't match
// … T+15 (or 16) -> instant, times match (or almost)
// Nothing for 30s -> 5s, times match
header('Cache-Control: s-max-age=10, stale-while-revalidate=20');
sleep(5);
print('This is a slow test. ' . date('Y-m-d H:i:s'));
?>