使用Ajax的PHP变量到PHP不起作用

I am new to using Ajax and am getting the success message here but there is an error that the variable is not defined.

//Variable TotalPoints is already set
$.ajax({
    type: "POST",
    url: 'save_pts.php',
    data: {'TotalPoints' : Allpts},
    success: function(data)
    {
        alert ('Success'):
    }
});

<?php // save_pts.php   
    $Allpts = $_POST['TotalPoints'];
?>
<script>
    alert("<?php echo $Allpts ?>");
</script>

Your code should be like this:

$.ajax({
type: "POST",
url: 'save_pts.php',
data: {TotalPoints : Allpts},
success: function(data)
{
    alert (data):
}
});

PHP CODE(save_pts.php)

<?php 
$Allpts = "No post value";
if(isset($_POST['TotalPoints'])){  
$Allpts = $_POST['TotalPoints'];
}
echo $Allpts;
?>

TotalPoints is the key of the property. The value is Allpts, so you need to create a variable with the name Allpts:

var Allpts = 10;