title: How to make particular sub-select options appear when the value of the parent select option is changed
body: <p>I am creating Select option.</p>
  
  <p>Example: It's a sort of code I will be using for categories. When user selects on of the parent select's option another select option to be displayed as sub category of that category.</p>
  
  <p><strong>My code:</strong></p>
  
  <pre><code>&lt;form action="&lt;?php echo $_SERVER['PHP_SELF']; ?&gt;" method="post"&gt;
  &lt;select&gt;
  &lt;option value="car"&gt;car&lt;/option&gt;
  &lt;option value="phone"&gt;phone&lt;/option&gt;
  &lt;option value="tv"&gt;tv&lt;/option&gt;
  &lt;/select&gt;
  
  &lt;select&gt;
  &lt;option value="toyota"&gt;toyota&lt;/option&gt;
  &lt;option value="nissan"&gt;nissan&lt;/option&gt;
  &lt;option value="bmw"&gt;BMW&lt;/option&gt;
  &lt;/select&gt;
  &lt;/form&gt;
  </code></pre>