title: What does 'P' stand for in the DateInterval format?
body: <p>Consider the following example quoted from <a href="http://www.php.net/manual/en/datetime.sub.php" rel="noreferrer">php manual for DateTime</a></p>
  
  <pre><code>&lt;?php
    $date = new DateTime('2000-01-20');
    $date-&gt;sub(new DateInterval('P10D'));
    echo $date-&gt;format('Y-m-d') . "\n";
  ?&gt;
  </code></pre>
  
  <p>'D' is for days, what does the 'P' stand for in that formatting?</p>