Becarful using this if you are trying to switch out of an existing session rather than load one into a clean slate.
session_decode doesn't destroy the existing session data, it will over write it if there is a session variable of the same name, but if the names don't clash the existing session variables will hang around.
I have yet to find a better solution than
session_destroy()
session_start()
session_decode(....);
-----------------------------------------
To explain what I'm talking about
<?
session_start();
$a = 5;
session_register('a');
session_decode("<session that doesn't have a as a session variable>");
print (session_is_registered('a') ? $a : 'Not Registered' );
?>
The above code will print '5' as $a hasn't been destroyed or even unregistered by the session_decode
