ten index je poradove cislo tvojho formularu ktore ti doplni do jeho id
takze to bude vyzerat nejak takto
<form id="aa-0">
...
</form>
<form id="aa-1">
...
</form>
teda ked budes volat onchange="document.getElementById('aa-1').submit();"> tak submitnes ten spravny formular
a k tomu header('location...')
Remember that header() must be called before any actual output is sent, either by normal HTML tags, blank lines in a file, or from PHP. It is a very common error to read code with include, or require, functions, or another file access function, and have spaces or empty lines that are output before header() is called. The same problem exists when using a single PHP/HTML file.
http://www.php.net/manual/en/function.header.phptakze ti to funguje len zazrakom a ja osobne pochybujem ze ti to funguje spravne.