Leggere il manuale e gli esempi gentilmente postati da neroux no eh?!?!

Da manuale:

codice:
Example #1 setcookie() send example

<?php
$value = 'something from somewhere';

setcookie("TestCookie", $value);
setcookie("TestCookie", $value, time()+3600);  /* expire in 1 hour */
setcookie("TestCookie", $value, time()+3600, "/~rasmus/", "example.com", 1);
?>
codice:
Example #2 setcookie() delete example

When deleting a cookie you should assure that the expiration date is in the past, 
to trigger the removal mechanism in your browser. 
Examples follow how to delete cookies sent in previous example:
<?php
// set the expiration date to one hour ago
setcookie ("TestCookie", "", time() - 3600);
setcookie ("TestCookie", "", time() - 3600, "/~rasmus/", "example.com", 1);
?>