title: Validate input is in YYYY-MM-DD HH:mm:ss format?
body: <p>I'm writing a PHP script that takes in a user entered value that MUST be something like this.</p>
  
  <pre><code>2011-06-17 00:00:00
  ...
  2011-06-17 23:59:59
  </code></pre>
  
  <p>How do I verify that it is indeed a correct input?</p>