title: Array_filter and empty()
body: <blockquote>
    <p>Warning: array_filter() expects parameter 2 to be a valid callback,
    function 'empty' not found or invalid function name....</p>
  </blockquote>
  
  <p>Why is empty considered a invalid callback?</p>
  
  <pre><code>$arr = array_filter($arr, 'empty');
  </code></pre>
  
  <p>This works: <code>if(empty($arr['foo'])) die();</code></p>