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The script isn't entirely necessary, I think, because it's most likely used to store data into a cookie, which in turn is read before displaying the next section of the form. At least for me, this would be easier to do in PHP.
You might want to search for some Javascript cookie examples, or if you'd like, I can post an example of the PHP version.
nein... a PHP example would only work after the form was submitted. If you want it to happen as soon as the user clicks a checkbox, you're going to need javascript. I'll try to post an example later. It shouldn't be very difficult.
Varland might be correct, but wouldn't it be better if there would be an option like Check here if the billing address is the same as your shipping address, with the billing address clearly visible at the time? That wouldn't really require any preprocessing or cookies at all. Then, if the checkbox is checked or there is no information on the shipping address fields, one could assume that the addresses are one and the same.
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