Client side:
1. How do you send the selected value to the server? Is there <form> tag or your Javascript does it?
2. How do you identify the name of the parameter? Does your <select> tag has a 'name' attribute or your Javsascript does it?
Server side:
3. Did you try reading the selected value via
$_POST?
4. If you did the step 3, did you try using
if conditions or
switch conditions to check the selected value?
5. If you did all these steps, what all you need is to set a header "Location" header to the redirect path as in
header('Location: php1.php');