Ajax发布到PHP是空的

I am trying to make an AJAX post to php but for some reason my PHP says that the post is empty. I don't get an error message from PHP or in my console. The alert(data) at the end returns all the HTML in index.php and the var_dump(post) returns empty. first is the console.log(aantal) the second is console.log(data) End of the console.log

script.js

$(document).ready(function () {

    var date = "date";
    var begin = "begin";
    var eind = "eind";
    var aantal = "aantal";


    $('#datum').change(function () {
        date = $("#datum").val();
        console.log(date);
    });
    $('#beginTijd').change(function () {
        begin = ($(this).val());
        console.log(begin);
    });
    $('#Tijdsduur').change(function () {
        eind = ($(this).val());
        console.log(eind);
    });
    $('#aantalSloepen').change(function () {
        aantal = ($(this).val());
        console.log(aantal);
        $.ajax({
            type: "POST",
            url: "index.php",
            data: {
                date: begin,
                quantity: aantal
            },
            success: function (data) {
                alert(data);
                console.log(data);
            }
        });
    });
});

index.php

<?php
var_dump($_POST);
$value1 = "";
$value2 = "";
if (isset($_POST['date'])) {
    echo "Yes, mail is set";
    $value1 = $_POST['date'];
    $value2 = $_POST['quantity'];
} else {
    echo "No, mail is not set";
}
echo $value1;
echo $value2;