About dropdown combo

[saraa]

hi all..please let me know..

I have 2 tables..category(cat_id,cat_code,cat_desc) and item(item_id,item_code,item_desc,cat_code).One category have many items.
There are 2 drop down combos in my form.for category and item.if I select the category, just items under category will listed in item's drop down combo...
Here is my codes:

for category:

<td><select name="cat_code">
            <?
                $sqlst = mysql_query("SELECT cat_code,cat_desc FROM category  ORDER BY cat_desc");
                while($cat = mysql_fetch_array($sqlst)){ ?>
                <option  value="<? echo $cat['cat_code'] ?>"><? echo $cat['cat_desc'] ?>
                </option>
                <? } ?>
              </select></td>

For Item:I dont know how and where to assign a variable for $cat_code

<td><select name="item_code">
            <?
                $sqlst = mysql_query("SELECT item_code,item_desc FROM item WHERE cat_code='$cat_code'");
                while($item = mysql_fetch_array($sqlst)){ ?>
                <option  value="<? echo $item['item_code'] ?>"><? echo $item['item_desc'] ?>
                </option>
                <? } ?>
              </select></td>

Thnks in advance