For your SQL,
I would have a customer_order table that had most of the fields from your Order table except productID. Then I would have an ordered_items table that had the details of a particular item that was ordered and an OrderID. So to figure out what items a particular order contained you would use an SQL statement like
Code:
SELECT * FROM Ordered_Items WHERE OrderID = 10
That way each item a user orders will be in a seperate row in Ordered_ITems but the entire Order can be referenced easily.
For the PHP,
I could be wrong but I think each tickbox in the select form returns a true/false value as to whether it is ticked, that you can use with a loop to add items.