How to use credentials?

I’ve tried putting the access keys in wp-config, and also in the database. From what I’ve understood of the code, they should be now be environment credentials and don’t need to be specified when I do something like:

$client = SnsClient::factory(array(
    'region'  => '<region name>'
));

But that doesn’t work, I get:

PHP Fatal error: Uncaught Aws\Common\Exception\InstanceProfileCredentialsException: Error retrieving credentials from the instance profile metadata server. When you are not running inside of Amazon EC2, you must provide your AWS access key ID and secret access key in the “key” and “secret” options when creating a client or provide an instantiated Aws\Common\Credentials\CredentialsInterface object.

What is the expected way to use the credentials?