im not sure what you mean, do you want to pass a variable name and have that variable set.
// url: /file.php?user=$user
$user = 'Bob';
echo $_GET['user']; // should echo 'Bob';
If thats what you want to do, there are different ways you can go about this. You could use eval(), however that could be very risky. If you just want the one variable, I would use a basic str_replace and replace the value. The only thing I wonder, is if have user set, you could just check if it is set and if it is set $user = username.
If thats not what you want to do, please could you explain a little more.