Shopping cart handling. Back button

[Anon]

Hi, I can't figure out how to deal with the one problem.
I am building a shopping cart where the user adds items to his shopping
cart and shippes some to himself and others to someone else as a gift.

I do the following thing, First let the user fillout his info and I ask him to check which items does he want to send to himself. Once he hit submit button I do the following.
1) I process which poduct does he want to ship to himself. Then
I remove those products from the shopping cart.
2) Then if he has items in his shopping cart remaining I display address form and checkboxes asking user to check all items that he want
to ship to this address.

      then I repeat step 2 until the user has no more items in his shopping cart.
      when he has no items I will display billing page.

Here is my problem if user hits back button in the browser and then enters submit
button I have not items in the shoppingcart since I removed them from his shopping cart the first time he processed his form.

So how do I deal with back button in the browser. I have no idea how to it
when I have multiple forms.

Thanks

[Anon]

The first thought that comes to mind is to maintain one or more "state" variables that track what the customer has and has not done during their session. Given the inconsistencies I've encountered with PHP4 sessions, I use database tables to retain this information. The upside of using a table is that, if I store the data in a logical manner, it can provide an "audit trail" for later reporting purposes.

Once you are maintaining "state" information, you place code at the beginning of each page that checks particular state data and intelligently displays the current form or redirects the user to a form where you can advise them as to what makes sense next.