there is a problem with it but I got one of the newer versions of PHP and found a way around it, setting the var is the same way as you already did it, BUT viewing it is a little different
$test = $_SESSION['test'];
echo "test: " . $test;
that should solve the problem, tedious but works for me, same problem I had if I had some session variables that I wanted to put in with form info, make a normal var first, seems to do the trick.
use the new session info and forget $session_register, don't use it anymore, read about sessions in the new help file that comes out with PHP lot of great info in it