Problem of destroy some session variable

[admin]

Dear all,

When I want to destroy several session variables, I use the following codes

<?
session_unregister("userid");
session_unregister("class");
session_unregister("email");
session_unregister("username");
session_unregister("admin");
header ("Location: all.htm");

exit();

?>
However, after we run the codes, these session variables still exist. Please give me solution, thanks

Sincerely yours
Simon