My cookies are expiring at the end of the session

[mr.pita]

I'm using the following code to set a cookie:

   $expire = time()+60*60*24*90;
   $cookiestring = "mycookievalue";
   setcookie("cookiname", $cookiestring, $expire, "/", ".mydomain.com", 0, 0); 

The thing is, that the cookie expires at the close of the browser--in every browser. If I go see the cookie in Firefox, it even says that it expires at the end of the session.

I really don't understand what I'm doing wrong.