Php de iyi olduğum sayılmaz ama şöyle bişi buldum senin için

Deleting cookies

The easiest way you can use this script is to link it to a "Log off" button or link:

<?
//Killing the cookie:
$cookie_name="myCookie";

//here we assign a "0" value to the cookie, i.e. disabling the cookie:
$cookie_value="";

//When deleting a cookie you should assure that the expiration date is in the past,
//to trigger the removal mechanism in your browser.
$cookie_expire=time()-60;

$cookie_domain="";
setcookie($cookie_name, $cookie_value, $cookie_expire, "/", $cookie_domain,0);

//re-direct to login screen (or any other you like):
header( "Location:login.html");
exit;
?>

header( "Location:login.html"); Login html kısmını kendi login alanin olarak değiştir

//Killing the cookie:
$cookie_name="myCookie"; my cookie yerine senin cookie adın

Gerisi de belli işte bir log off butonuna link ver.