process transaction function paypal - php

Question

I am building an e-commerce application through video tuition.
I have a process_transaction function that parses PayPal tx params from the URL upon a successful purchase and populates an 'orders' table in phpMyAdmin 4.8.0.
Previously, if your refreshed my thank_you.php page the orders continue to populate my 'orders' table (as there was previously no session destroy built into the aforementioned function).
The problem is, the code suggested to prevent such by our tutor for this function does not work. When run, the 'orders' table is no longer populating from the transactions nor is session_destroy operational.
I was wondering what is wrong with the function.
thank_you.php
<?php require_once("../resources/config.php"); ?>
<?php require_once("../resources/cart.php"); ?>
<?php include('../resources/templates/front/header.php') ?>
<?php process_transaction(); ?>
<!-- Page Content -->
<div class="container">
<h1 class="text-center">THANK YOU</h1>
<h3>Nippon will get your products shipped today</h3>
</div>
<!-- /.container -->
<?php include('../resources/templates/front/footer.php') ?>
cart.php (not working properly - does not persist orders in db, or session destroy)
<?php
function process_transaction() {
if(isset($_GET['tx'])) {
$amount = $_GET['amt']; // get amount
$currency = $_GET['cc'];
$transaction = $_GET['tx']; //
$status = $_GET['st']; // get status ie completed
$total = 0;
$item_quantity = 0;
foreach ($_SESSION as $name => $value) {
if($value > 0 ) {
if (substr($name, 0, 8) == "product_") {
$length = strlen($name);
$id = substr($name, 8, $length);
$send_order = query("INSERT INTO orders (order_amount,order_transaction,order_status,order_currency) VALUES('{$amount}','{$transaction}','{$status}','{$currency}')");
$last_id = last_id();
confirm($send_order);
$query = query("SELECT * FROM products WHERE product_id = " . escape_string($id) . " ");
confirm($query);
while($row = fetch_array($query)) {
$product_price = $row['product_price'];
$product_title = $row['product_title'];
$sub = $row['product_price'] * $value;
$item_quantity += $value;
$insert_report = query("INSERT INTO reports (product_id, order_id, product_title, product_price,product_quantity) VALUES ('{$id}','{$last_id}','{$product_title}','{$product_price}','{$value}')");
confirm($insert_report);
}
$total += $sub;
echo $item_quantity;
}
}
}
session_destroy();
} else {
redirect("index.php");
}
}
functions.php
function query($sql) {
global $connection;
return mysqli_query($connection, $sql);
}
function fetch_array($result) {
return mysqli_fetch_array($result);
}
function escape_string($string) {
global $connection;
return mysqli_real_escape_string($connection,$string);
}