In a project I work on I ran into something I would say
is a bug..
When I call the page with ?debug=3 the session var 'test'
gets the value "howdy" and gets stored (no need to call
session_register('test'); (odd?)). After this there seem
to be no way of unsetting/removing this session var..
At least non of the methods below works. Bug?
unset in #2 (see below) removes the var, so it doesn't
show up in the var_dump (#6), but upon calling the page again with ?debug=2 (only show sess-vars), it shows up
again.

At the same: $_GETand $_SERVER is displayed in phpinfo, but not $_SESSION. Another bug?

In php.ini:
register_globals = On

Script:
// #1
if ($debug==3)
  $_SESSION['test'] = "howdy";

// #2
if ($debug==4) 
  unset($_SESSION['test']);

// #3
if ($debug==5) 
  unset($test);

// #4
if ($debug==6) 
  session_unregister($test);

// #5
if ($debug==7) 
  session_unregister($_SESSION['test']);

// #6
if ($debug>1)
  var_dump($_SESSION);