Domain a use AJAX post though domain B then domain B set cookie self to login

<script>
    $(document).ready(function($) { 
        $.ajax({
            type: "POST",
            url: "http://domainB/setcookie.php",
            data:{
                'id': 'value'
            },
            success: function(data){
                console.log(data);
            }
        });
    });
</script>

<?php

 setcookie("u_id", $_POST["id"], 0, '/');

?>

The result of $_COOKIE["u_id"] is NULL EMPTY. I want $_COOKIE["u_id"] is "value".