Categories box

[Tao]

In my shop, I require customers to select a category which they desire before they go to osCommerce. So the link is like as:

 

mydomain/catalog/index.php?cPath=28

 

I want the Categories box displays the selected category only (include its sub categories and the products under this selected category). In this case, the selected category which ID is 28 and its sub items can be displayed in the category box while other categories can not be shown up during the whole session. To solve this problem, which php files do I need to modify, and how? Thanks in advance.