title: How to remember checkbox state when default state is checked?
body: <p>I have a form with a check box.</p>
  
  <p>The check box has three "states":</p>
  
  <ol>
  <li>Checked as default.</li>
  <li>Unchecked by user.</li>
  <li>Checked <em>again</em> by user.</li>
  </ol>
  
  <p>How would I do something like that? If I use <code>!isset()</code>, I can uncheck the checkbox but it doesn't distinguish if it was unchecked by the user or if it's suppose to be checked as default.</p>
  
  <p>I want to use this to refill  the last values of the form inputs whenever there are form errors on submit.</p>
  
  <p>It's easy to do when the default state of the check box is unchecked. I can then just do:</p>
  
  <pre><code>if (isset($_POST['checkbox'])) echo ' checked'; //check box has been checked
  </code></pre>