title: PHP Unit Test: How to test a class that implements an interface from another package?
body: <p>I am trying to test a class that implements an interface that is defined outside the scope of the classes package (i.e. I pull it in as a dependency using composer during integration).</p>
  
  <p>I would like to test this concrete class without having to pull in the interface it depends on. Is there anyway to test this in php unit test?</p>
  
  <p>At the moment I can't even instantiate an instance as the interface is obviously not found.</p>