category management using MySQL and PHP

Hi All, I am working on small web project based on PHP & MySQL where user is looking for items in certain categories and he specifies the category in search box, the search result should return all items from that category & sub-categories of it. I thought of defining a unique identifier to each category and use them in search but it may work well only for base categories, for hierarchical/sub categories I doubt it will make it simple. Any hints on how can I solve this? Any pointers would be greatly helpful. Thanks in advance, Pravin