Help display 2nd Dropdown list when selection is made (from first drop down list)

[chrisj]

I am trying to modify a existing web script's Upload Form where, currently you choose a Category...etc. and Submit. When a Category is chosen, I'd like another dropdown list of choices to display. Here is the code currently:

<div class="form-group">
<label class="col-md-12" for="category_id">{{LANG category}}</label>
<div class="col-md-12">
<select name="category_id" id="category_id" class="form-control">
<?php foreach($pt->categories as $key => $category) {?>
<option value="<?php echo $key?>"><?php echo $category?></option>
<?php } ?>
</select>
</div>
</div>

Any guidance/assistance with this appreciated.