I've resorted to not using the form action $_SERVER["PHP_SELF"] in WordPress and I'm instead using the relative path of the url, so for example if the first page is http://wordpress-site.com/form1 then I'd use /form1 for the first form page, /form2 (or whatever it's called) for the second and so on, so that it submits to itself and can continue the chain.
I'd really like to know why WordPress resolves PHP_SELF as index; but that's not something that I'm going to spend a lot of time on.
I'd really like to know why WordPress resolves PHP_SELF as index; but that's not something that I'm going to spend a lot of time on.