PHP - Unset() cookies

<?php

    var_dump($_COOKIE['carrito']); //string(66) "a:1:{i:0;a:2:{s:10:"referencia";s:4:"ref3";s:6:"precio";s:1:"2";}}"

    unset($_COOKIE['carrito']);  //Notice: Undefined index: carrito in C:\xampp\htdocs\compra.php on line 4

    var_dump($_COOKIE['carrito']); //NULL 

?>

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

$_COOKIE["carrito"]="";