dependable dropdown in codeigniter

CI doesn't have functions for hiding or displaying elements dynamically after the page is loaded. That's something you need Javascript for.
If you want the selected value to make CI do something (server sided), then you need AJAX.

Jquery is a very easy and efficient way for basic Javascript functions as well as advanced AJAX requests.
Include a link to the Jquery framework in the head section of your page (or template).

In your view file, after the closing php tag, you put the jquery script, like this:

PHP Code:

<?php
  // php code in view
?>

<script>
$(document).ready(function(){
  //put your jquery functions here
});
</script>