What is PHPSESSID

Question

I'm experimenting with cookies. I also don't have any cookies with the name PHPSESSID. Do I require it? Can I take it off?

What is its "function"? 

if (count($_POST)) {

setcookie("TestCookie", htmlspecialchars($_POST['val']), time()+3600);
}

print_r($_COOKIE);

Prints:

Array
(
    [TestCookie] => blabla
    [PHPSESSID] => el4ukv0kqbvoirg7nkp4dncpk3
)