Web development -- 2

Prestashop modification: the field 'Select a quantity' should increase the number of pieces in multiples of the minimum quantity for that product. E.g.= The product named Blue pen has the minimum quantity of 8 pieces. If the customer click on the up arrow for increasing the quantity he will see 16, then 24, 32, 40 etc. and he can't have the chance to order a different quantity such as 10 pieces, for example. This criterion should work in every order review page, in the cart and in the main order page. So in everywhere.