会话变量不是用AJAX定义的 - Node.js

The Problem:

Hello, i am trying to call a php file from node.js with AJAX, but when i do, the session variables return undefined and the database doesn't update. However, the php file does work because i tested it with a link in the browser. Unfortunately, i can not just define the session variables as data for AJAX because i can not include php in javascript or html. (Node JS doesn't allow me to change my index.html to index.php).

What Works:

• The File: When i place the php file as a link in the browser, the database is correctly updated and everything works perfectly.
• AJAX: When i call the php file from AJAX, it does echo the success message but when i changed the success message to one of the session variables, it returns blank. (This is how i found out that the variables are the problem)

The Code:

PHP:

subtract5.php

<?php
header('Access-Control-Allow-Origin: http://cashballz.net:3000', false);
include 'mysql.php'; 
session_start(); 

$cash_amount = $_SESSION['cash_amount'];
$userid = $_SESSION['id'];
$_SESSION['cash_amount'] -= 0.05;

$mysql = new Mysql(); 

$result = $mysql->setCashAmount($cash_amount,$userid); 
if($result) 
{ 
echo "5 cents have been subtracted!"; 
} 
else 
{ 
session_start(); 
session_unset(); 
session_destroy(); 
}
?>

mysql.php

<?php
class Mysql 
{ 
protected $dsn; 
protected $username; 
protected $password; 
public $db; 


function __construct() 
{ 

$this->dns= 'mysql:dbname=cashball_accounts;host=localhost;charset=utf8'; 
$this->username= 'myUser'; 
$this->password= 'myPass'; 
$this->db = new PDO($this->dns, $this->username, $this->password); 
} 


public function setCashAmount($cash_amount, $id) 
{ 
$sql = "UPDATE users SET cash_amount = :cash_amount - 0.05 WHERE id = :id"; 
$stmt = $this->db->prepare($sql); 
$stmt->bindParam(':cash_amount', $cash_amount, PDO::PARAM_STR); 
$stmt->bindParam(':id', $id, PDO::PARAM_STR); 
$result = $stmt->execute(); 
return $result; 
} 

} 
?>

JS:

app.js

   $.ajax({
    type: "POST",
    url: 'http://cashballz.net/game/5game/subtract5.php',
    data: {}, 
    success: function (data) {
        alert(data);
    }
});

Conclusion:

I was thinking that when i deploy the node server from index.php (my website's index file - not node's which is index.html) then i could send the php variables to like a json file with a global database and access that from node, but i tried to research it and have no idea how that works.

I am a beginner to PHP so i am open to many different solutions!

P.S. The most secure solution is preferred because my website uses real money.

Thanks for helping out!