[Andreas]

= is assignment, == is comparison
So the else if below thatsite will never be executed, eg. is useless.

In other words:
The code you posted is equivalent to

PHP Code:

if ($scr == 1)
{
    header ("Location: http://thissite");
}
else
{
    $id = $_GET['id'];
    header("Location: http://thatsite");
}