Remeber that javascript is running on the client PC and PHP is on the server.
Perhaps the easiest way to communicate between the two is for your JS script to put the value of sI into a hidden form field,
<input type="hidden" name="sI" value=""> // value set by script
then submit the form.
Your PHP script then picks up the value with
$sI = $GET["sI"] or
$sI = $POST["sI"]
depending on your forms method.
hth