The first thing to do is pass in your credentials and instantiate a Rackspace client:

<?php

require 'vendor/autoload.php';

use OpenCloud\Rackspace;

$client = new Rackspace(Rackspace::US_IDENTITY_ENDPOINT, array(
  'username' => '{username}',
  'apiKey'   => '{apiKey}',
));

Now, set up the Auto Scale service:

$service = $client->computeService('{catalogName}', '{region}', '{urlType}');

{catalogName} is the name of the service, as it appears in the service catalog. For Rackspace users, a default will be provided if you pass null in for this argument. For OpenStack users, you cannot do this: you must instead set your own value since it can depend on your environment setup.

{region} is the Compute region the service will operate in. For Rackspace users, you can select one of the following from the supported regions page.

{urlType} is the type of URL to use, depending on what endpoints your catalog provides. For Rackspace, you may use either internalURL or publicURL. The former will execute HTTP transactions over the internal network configured for your service, possibly reducing latency and the overall bandwidth cost - the caveat is that all of your resources must be in same region. publicURL, however, which is the default, will operate over the public Internet and is to be used for multi-region installations.