我正在使用serialize，但它没有返回结果，它显示错误
注意:C:\xampp\htdocs\ecommerce\placeorderajax.php行2中的未定义索引:firstname
这是我的html代码

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$('.place-order').click(function() {
  //  alert('Clicked');
  var form = $('#checkout-form').serialize();
  console.log(form)
  $.ajax({
    url: 'placeorderajax.php',
    type: 'POST',
    data: {
      data: form
    },
    success: function(response) {
      alert(response);
      console.log(response);
    }
  });
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<form action="" id="checkout-form" class="checkout-form" name="checkout-form">
  <div class="col-md-6 col-12 mb-20">
    <label>First Name*</label>
    <input type="text" name="firstname" id="firstname" placeholder="First Name">
  </div>
  <span class="place-order btn btn-lg btn-round">Place order</span>
</form>

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这是我的php代码

<?php 
$fname = $_POST['firstname'];
echo $fname;
?>