It should be possible to use $GET and $POST in same form
if the $_GET is in a link, URL.
As far as I know.
But if you use same variable name for GET as for POST
then the GET will usually be lost.
But if you put it into
<form action="index.php?id=7" method="POST">
I really dont know what is happening.
Better to put it in a plain normal <a href="....">