title: Stub for formal parameter passed by reference in PHPSpec/Prophecy
body: <p>I have a method in PHP which looks like this:</p>
  
  <pre><code>&lt;?php
  class SomeClass
  {
      public function isEntitledToDiscount(GuestModel $who, $onWhat, &amp;$discountAmount = null)
      {
          $discountAmount = 10;
          return true;
      }
  }
  </code></pre>
  
  <p>Is there a way to stub <code>$discountAmount</code> in PHPSpec? Let's say I have a class I'm testing, and I've injected <code>$someService</code> via constructor. In my spec I use:</p>
  
  <pre><code>&lt;?php
  $someService-&gt;isEntitledToDiscount($guest, $ticket, $discountAmount)-&gt;willReturn(true);
  </code></pre>
  
  <p>which creates stub for method return value. How can I do this with the <code>$discountAmount</code> formal parameter?</p>