Tiens ca peut peut etre t'éclairer ?
Note when setting "array cookies" that a separate cookie is set for each element of the array.
On high traffic sites, this can substantially increase the size of subsequent HTTP requests from clients (including requests for static content on the same domain).
More importantly though, the cookie specification says that browsers need only accept 20 cookies per domain. This limit is increased to 50 by Firefox, and to 30 by Opera, but IE6 and IE7 enforce the limit of 20 cookie per domain. Any cookies beyond this limit will either knock out an older cookie or be ignored/rejected by the browser.
Tiré de http://fr.php.net/setcookie
