title: REST API best HTTP Status response for illegal operation
body: <p>I'm creating a REST API in PHP.</p>
  <p>When the client may try to perform an action, which is unavailable, for example, it tries to change a property of the resource which is not passable: for example, tries to change the value of the &quot;country&quot; property to &quot;Julius Caesar&quot;:</p>
  <ol>
  <li>What HTTP status code should I send back with the response? I'm speculating between <code>403</code> and <code>409</code>.</li>
  <li>I don't know if <code>403 Forbidden</code> is only related to user permissions or can I use for this purpose?</li>
  <li>In what situation should I use <code>409 Conflict</code>?</li>
  <li>To summarize what is the proper HTTP response status to an illegal operation?</li>
  </ol>