<?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"]="";