how to search a subcatagory without click a submit button

[lukelee]

I want to search a product, there is 2 selects, first one is category, second one is sub category. when people select a category, the sub category under this category will come out. does anyone know how to do this?

 

<form method="post" action="<?php echo $_SERVER['PHP_SELF']; ?>" enctype="multipart/form-data">
  <table border="0" class="catagory" >
    <tr><td><select name="firstlevel" style="width:180px;">
      <option value="<?php echo $row['title']; ?>"><?php echo $row['title']; ?></option>
      </select></td>
      <td><select name="secondlevel" style="width:160px;">
      <option value="<?php echo $row['sub_title']; ?>"><?php echo $row['sub_title']; ?></option>
      </select></td>
      <td><input type="submit" name="submit" id="submit" value="Submit" /></td>
    </tr>
  </table>
</form>