问putObeject中ACL的所有可选值是什么？

$result = $cosClient->putObject(array(
        'Bucket' => $bucket,
        'Key' => $key,
        'Body' => fopen($local_path, 'rb'),
        'ACL' => 'string',
        //ACL如何填写  有哪些
        ),
        'StorageClass' => 'string'));