select category with fewest products

[HDFilmMaker2112]

I'm looking to know how to tweak this query so it finds the one category with the fewest products in it.

 

mysql_fetch_array(mysql_query("SELECT COUNT(product_id) AS num FROM products"));

[HDFilmMaker2112]

I basically need something like this:

mysql_fetch_array(mysql_query("SELECT COUNT(product_id) AS num FROM products 
WHERE product_cat=(SELECT product_cat FROM products WITH LEAST SUM(product_id))"));