Search In Categories

i need some hints how i can search in multiple categories. here is my code

<?php
$categories = get_categories(‘child_of=16&hide_empty=0’);
?>
<form action=”<?php bloginfo(‘wpurl’); ?>” method=”get”>
<select name=”cat” id=”cat”>
<option selected=”selected”>Select a Category</option>
<?php
foreach($categories as $category){
echo ‘<option value=”‘.$category->term_id.'”>’.$category->name.'</option>’;
}
?>
</select>
<input type=”text” value=”” name=”s” />
<input type=”submit” value=”Go!” />
</form>

currently it’s working for single category (dropdown). any clue?

[category1] – [category2] [search box] [go button]