删除会话cookie

Why is it that when I call:

<?php

setcookie("PHPSESSID", "", time() - 3600);

?>

on a session cookie it doesn't get deleted but if I were to call it on another cookie it does get destroyed?

This works perfectly fine:

<?php

setcookie("tom", "tom", time() - 3600);

?>

but the above example with the session cookie doesn't.

I have tried this on google chrome and firefox and still get the same result when looking at the header files.

This is the output for session cookies:

Array
(
    [_GET] => Array
        (
        )

    [_POST] => Array
        (
        )

    [_COOKIE] => Array
        (
            [PHPSESSID] => v61njiklq3kd5koc6i8i7otab7
        )

    [_FILES] => Array
        (
        )

    [GLOBALS] => Array
 *RECURSION*
)