Illustrates how to retrieve an S3 hosted asset ETag (likely MD5 sum) from a Laravel application.

etag.php

/**
 * Retrieves the ETag for the given S3 key.
 * 
 * @param $key The file key. This is the URL of the object relative to the bucket. It can be found through the S3 portal.
 * @return string The ETag 
 * @throws S3Exception
 */
public function getEtag($key)
{
    $adapter = Storage::disk('s3')->getAdapter();
    $client = $adapter->getClient();
    $object = $client->headObject([
        'Bucket' => config('filesystems.disks.s3.bucket'),
        'Key' => $key
    ]);

    return $object->get('ETag');
}