Two combobox Database

[NeedZoom]

I have two combobox  : "<select>   </select>" from database.

one  combobox depends on another combobox.I have a problem with the connection between the two.

 

The first combobx : 

in tpl file :

<select onchange=" " name="id">
    <option value="Select">Select</option> 
     {foreach from=$categories item=categorie}
          <option value="$id = {$categorie.id_category}" {if $smarty.id}selected = "selected"{/if}>{$categorie.id_category}</option>
         {/foreach}
    </select>

in Controller.php

public function initContent()
  {parent::initContent();
   $categories = Db::getInstance()->ExecuteS('
   //my sql 
   ');
     $this->context->smarty->assign('categories',$categories);
     //$this->context->smarty->assign('souscategories',$souscategories);
     $this->setTemplate(_PS_THEME_DIR_.'mytpl.tpl');
  }

The seconde combobx:

 

When the customer choose the category, retrieve the id of this class and run the  seconde  query

$id=Tools::getValue('id');
$souscategories = Db::getInstance()->ExecuteS('

SELECT * 

FROM table where n = '.$id.'

');

 

Thanks to give some help , idea solve it

Edited January 30, 2014 by NeedZoom (see edit history)

[NeedZoom]

Now the second combobox work but i should clik the botton 

in my file TPL i use :

{if $smarty.post.id}selected = "selected"{/if}> 

in the fil  controller.php:

if($id=Tools::getValue('id')){
$souscategories = Db::getInstance()->ExecuteS(
SELECT *
FROM table
where champs = '.$id.' and ');
$this->context->smarty->assign('souscategories',$souscategories);
} 

I don't any idea to slove this problem , PLZ help me