How to Search the product within the top lovel category in Magento

The default magento supports to search the products based the search keyword. if we want to search the products within the specific category we need to change the catalog search template under teample/catalogsearch/form.mini.html

here is the script to search the products within the specific category

<?php
$category = Mage::getModel('catalog/category');
if(is_object(Mage::registry('current_category'))){
    $current_category_path=Mage::registry('current_category')->getPathIds();
}else{
    $current_category_path = array();
}
$category->load(Mage::app()->getStore()->getRootCategoryId());
$children_string = $category->getChildren();
$children = explode(',',$children_string);
$extra_options='';
foreach($children as $c){
    $selected = (in_array($c, $current_category_path))?'SELECTED':'';
    $extra_options.= '<option value="' . $c . '" ' . $selected . '>' . $category->load($c)->getName() . '</option>' . "\n";
}
?>
<form id="search_mini_form" action="<?php echo $this->helper('catalogSearch')->getResultUrl() ?>" method="get">
    <fieldset>
        <legend><?php echo $this->__('Search Site') ?></legend>
        <div class="mini-search">
            <input id="search" type="text" class="input-text" name="<?php echo $this->helper('catalogSearch')->getQueryParamName() ?>" value="<?php echo $this->helper('catalogSearch')->getEscapedQueryText() ?>" />
            <select name="cat" id="cat" class="input-text">
            <option value="">All Categories</option>
            <?= $extra_options ?>
           </select>
            <input type="submit" value="Go" style="border: 1px solid #808080;" alt="<?php echo $this->__('Search') ?>" />
            <div id="search_autocomplete" class="search-autocomplete"></div>
            <script type="text/javascript">
            //<![CDATA[
                var searchForm = new Varien.searchForm('search_mini_form', 'search', '<?php echo $this->__('search site...') ?>');
                searchForm.initAutocomplete('<?php echo $this->helper('catalogSearch')->getSuggestUrl() ?>', 'search_autocomplete');
            //]]>
            </script>
        </div>
    </fieldset>
</form>