Helo, first time caller, long time supporter,

Below I have examples of portions of my login script and logout script. For some people (like myself), logging out works just fine, but for others, it do not: they just stay logged in. I've tried a lot of things, and read even more about it and by all accounts, the below code SHOULD work all of the time, does anyone have any suggestions? Please recognize that I understand better by example as I'm only on the cusp of becoming a full-fledged novice. Thank you very much in advance.

LOGIN PAGE/SCRIPT:

(I don't want to take up a lot of space on this page, so check this link to see the code: http://publicecology.org/login.txt)

LOGOUT PAGE/SCRIPT:

Code:
<?
setcookie("SITEMANAGER_ID","-",time()-2592000);
setcookie("SITEMANAGER_TIME","-",time()-2592000);
setcookie("SITEMANAGER_CID","-",time()-2592000);
setcookie("SITEMANAGER_USER","-",time()-2592000);
header("Location: index.html");
?>