title: ?: operator (the 'Elvis operator') in PHP
body: <p>I saw this today in some PHP code:</p>
  
  <pre><code>$items = $items ?: $this-&gt;_handle-&gt;result('next', $this-&gt;_result, $this);
  </code></pre>
  
  <p>I'm not familiar with the <code>?:</code> operator being used here. It looks like a ternary operator, but the expression to evaluate to if the predicate is true has been omitted. What does it mean?</p>