title: What should be the result of !$a - $a
body: <p>I have this code:</p>
  
  <pre><code>$a = 1;
  $a = !$a - $a;
  echo $a;
  </code></pre>
  
  <p>I was thinking of <code>!0</code> - kidding, but the output is <code>-1</code>. Unfortunately I can't understand why the result is <code>-1</code>. It would be great if somebody explains.</p>