How to test license validation via REST API

 
I am writing test case using PHPUnit to check the above method. I am able to cover all cases except one case in which a domain license should return true in is_valid xml node.

The REST API is so secured that it does not accept request from IPs that are not listed in their whitelist. And if someone is making a request from a non-whitelisted IP the API returns a false value for is_valid (and this is how I am covering the case for false)

I know this can be done using a mock object but I am not really sure how to write a mock object that can cover the case where a domain name is valid. Can someone please help?

Thanks in advance