Cookie vulnerability in Alguest guestbook (PHP)

["MOD" <br014c1155 () blueyonder co uk>]

Alguest is a guestbook programmed in PHP, there is a major flaw in it which
enables any user to access the admin panel. The script can be downloaded
from
http://www.hotscripts.com/cgi-bin/dload.cgi?ID=14105
It has a flaw in which cookie data isn't properly checked for administrator
rights (username, password), it only checks if the cookie is present
"elseif(isset($admin))" Therefore anyone can just create a cookie and gain
access to administrator privledges.
A solution might be this "elseif(isset($HTTP_COOKIE_VARS['admin'] ==
$password && $username))" but I haven't tested it so I can not guarantee it.