title: How to make PHPunit return nonzero exit status on warnings
body: <p>When invoking PHPunit on some tests which fail with warnings, I get:</p>
  
  <pre><code>$ phpunit -c phpunit.xml --group app
  Warning - MongoCollection::insert(): expects parameter 1 to be an array or object, null given in ...
      &lt;more output&gt;
  
  OK, but incomplete or skipped tests!
  Tests: 17, Assertions: 81, Incomplete: 1.
  </code></pre>
  
  <p>One of the tests should fail, but it doesn't; PHPunit marks it as "incomplete".</p>
  
  <p>Let's check last exit status:</p>
  
  <pre><code>$ echo $?
  0
  </code></pre>
  
  <p>The config I am using is:</p>
  
  <pre><code>&lt;phpunit
      convertErrorsToExceptions="true"
      convertNoticesToExceptions="true"
      convertWarningsToExceptions="true"
      strict="true"
      stopOnError="true"
      stopOnFailure="true"
      stopOnIncomplete="true"
      stopOnSkipped="true"
  
      colors="true"
      bootstrap="bootstrap_phpunit.php"
      &gt;
  </code></pre>
  
  <p>Any idea how to force PHPunit to emit nonzero exit status in case of "incomplete" tests?</p>