Foreign key in php and mysql?

i have 3 tables products ,categories an pro_cat.
pro_cat has 2 foreign keys pro_id references to products->id and cat_id references to categories->id .
i want when i add new product to the products table to add the id of the product and the id of category in pro_cats table.
so i can retrieve it later ... how can i do it ?
my insert query to products

$query="INSERT INTO products (name,description,price