How to create Dynamic category and subcategory menus ?

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PHP

Product_category table

category_id ---- category_name
1 ------------------ CARS
2 -------------------TRUCKS


Product_list table

Product_id ----- category_id (foreign key)----- Product_name
1 ----------------- 2 -------------------------- Ford trucks
2 ----------------- 1 ------------------------- - Ferrari sports cars
3 ----------------- 1 ---------------------------- Honda coupes

above i have listed my database design... I would assume that a Product_category would contain mulitple subcategories. That means that there is a one-to-many relationship between the Product_category and Product_list tables. In which case, the category ID should be a foreign key in the Product_list table

....i have to create subcategory under a category... its like.. if i click CARS then it MUST show subcategories 1 Ferrari sports cars , 1 Honda coupes if i click TRUCKS it must show 2 Ford trucks... can anyone please help me please write down the php code Thank you