AJAX调用成功,但未在服务器上检索数据

I have an ajax call that passes data to another php file, createTest2.php, as below.

But the createTest2.php file throws error

"Notice: Undefined index: aaa in C:\xampp\htdocs\TestProj\Test\createTest2.php on line 2

I have no clue how to fix it.

caller.php

$(document).ready(function(){
    $("#button_submit").click(function() 
  {

    $.ajax({
      type:"POST",
      url:"createTest2.php",
      data:{aaa : "UNIT_TEST"},
      success:function()
      {
        alert("success");
      }
    });
 });
});

createTest2.php

$test_name = $_POST['aaa'];
    if you are using 
    $test_name = $_POST['aaa'];
 you have to call ajax like this
$(document).ready(function(){
    $("#button_submit").click(function() 
  {

    $.ajax({
      type:"POST",
      url:"createTest2.php",
     data:"aaa=UNIT_TEST",
      success:function()
      {
        alert("success");
      }
    });
 });
});

the main thing is that " data:"aaa=UNIT_TEST", "

Try this:

//sample.php
<?php
if(isset($_POST) && isset($_POST['aaa'])){
    echo json_encode("hello world! Your data was: " . $_POST['aaa']);
}
?>

//your client side page
<html>
<head>
    <title></title>
    <script src="http://code.jquery.com/jquery-1.9.1.min.js"></script>
    <script type="text/javascript">

        function send(){
            $.ajax({
                type:"POST",
                url: 'sample.php',
                data:{aaa:"UNIT_TEST"},
                dataType: 'json'
            }).done(function(data){
                alert(data);
            });
        }

    </script>
</head>
<body>
<a href="#" onclick="send();">Send</a>
</body>
</html>

Try this in your, for example, index.html:

<script>
$(document).ready(function(){
    $("#button_submit").click(function() 
  {

    $.ajax({
    global: false,
    type:"post",
    dataType: "html",
    cache: false,
    url: "createTest2.php",
    data: {aaa:'test'},
    success:function(html)   {    alert(html);  },
    error: function(e) {alert(e);}
    });
 });
});
</script>

And in your createTest2.php just put this to see if ajax calls are received:

<?php
echo $_POST['aaa'];

?>