Setcookie() pushes the cookie onto a stack (only on Apache, if I read the code right).  Using a stack instead of a FIFO causes the cookies to be sent to the browser in the reverse order that they are set in the script.  This is a problem when a script needs to clear a cookie and then may or may not set the cookie with a new value later in the script.  In the case where the cookie gets a new value, it is immediately cleared due to the reverse ordering.