将 ID 名称发送到 php 变量 （AJAX） 时未定义的数组键 [duplicate]

问：

我有一个错误

Warning: Undefined array key "data" in C:\xampp\htdocs\php\recipe-container.php on line 9 ($product = $_POST["data"];)
Warning: Undefined variable $response in C:\xampp\htdocs\php\recipe-container.php on line 24 (echo $response;)

我正在尝试通过AJAX发送单击的div的ID（具有“product”类的div）。Div ID 名称似乎显示在网络选项卡中，但在 PHP 代码中未定义。

//AJAX (head HTML tag)
$(function () { 
        $('.product').on('click', function (e) {
            var product = $(this).attr('id');
            
            $.ajax({
                type: "post",
                url: 'php/recipe-container.php',
                data: product,
                processData: false,
                contentType: false,
                success: function(response) {
                    $(".display_recipe").html(response);
                }
            });
        });
});

//HTML
<div class="product" id="pizza">pizza</div>
<div class="product" id="lasagna">lasagna</div>
<div class="product" id="sushi">sushi</div>
<div class="display_recipe">
  <?php
    require "php/recipe-container.php";
  ?>
</div>

//PHP (recipe-container.php)
<?php

function display(){
    $con = mysqli_connect("localhost", "root", "", "cookbook");
    echo "test";
    $product = $_POST["data"];
    $sql = "SELECT * FROM product WHERE name = '$product'";
    $res = mysqli_query($con,$sql);

while($row = mysqli_fetch_assoc($res)) {
    $name = $row['name'];
    $description = $row['description'];
    $date = $row['date'];

    $response = $name.'<br>';
    $response .= $description.'<br>';
    $response .= $date.'<br>';
    
}
echo $response;

mysqli_close($con);
}

if ($_SERVER['REQUEST_METHOD'] === 'POST'){ 
    display(); 
}

?>

我尝试使用 and for ，但没有用。现在它显示“测试”，但没有其他内容。我该如何解决？isset!empty$_POST["data"];