asynch transfer fails in AWS/s3 while synch transfer works

Question

I am trying to transfer a few files from an s3 bucket to my laptop. When I do the transfer synchronously, like this, it works fine: $manager = new \Aws\S3\Transfer($client, $source, $dest); $manager->transfer();

But when i try to do the same transfer aynch like this:

    $manager = new \Aws\S3\Transfer($client, $source, $dest);
    $promise = $manager->promise();

    $promise->then(function () {
        echo 'Done!';
    });
    $promise->otherwise(function ($reason) {
        echo 'Transfer failed: ';

    });

In this asynch case, the files are never transferred. The async code above is directly from AWS php SDK documentation. Is there something that they forgot to mention in the docs?