effect of updating data from multiple users sql php - php

Question

I have a question please,
I am making an online store.
It offers a product with an available quantity attribute
Let's imagine a certain product has 1 quantity available only.
2 users are logged in and added this element to cart.
They both at the same time checked out (I am trying to do this by splitting 2 browsers in the screen and pressing at the checkout button simultaneously.)
How can I manage that the product checkout is successful for only one user in this case?
if ($_SERVER["REQUEST_METHOD"] == "POST") {
if(isset($_POST['qty_array'])&&isset($_POST['ids_array'])){
$order_address=$_POST['addressCart'];
$ids_array=$_POST['ids_array']; //this is array of product ids in the cart
$qty_array=$_POST['qty_array']; // this is array of quantities in the cart
$date=date("Y-m-d H:i:s");
$j=0;
$x=0;
foreach($ids_array as $id){
$query = "SELECT available_quantity from products where product_id='$id' AND product_active=1";
$result = mysqli_query($db,$query);
$row = mysqli_fetch_array($result);
$qty1=$qty_array[$j];
if($row['available_quantity']==0 || $qty1>$row['available_quantity']){
$x++;
}
$j++;
}
if($x!=0){
$msg="unable_checkout";
echo $msg;
}
else{
$bool=mysqli_query($db,"INSERT INTO order_table(customer_id,order_date,order_address)
VALUES ('$customerID','$date','$order_address')") or die("".mysqli_error($db));
$order_id=mysqli_insert_id($db);
if($bool){
$k=0;
foreach($ids_array as $id){
$query2 = "SELECT * from products where product_id='$id' AND product_active=1";
$result2 = mysqli_query($db,$query2);
$row2 = mysqli_fetch_array($result2);
$qty_available=$row2['available_quantity'];
$qty=$qty_array[$k];
$unitprice=$row2['product_price'];
$discount=$row2['product_discount'];
if($qty<=$qty_available && isset($_SESSION['cartProds']) && $qty_available!=0 ){
mysqli_query($db,"INSERT INTO order_details(order_id,product_id,unitprice,discount,quantity)
VALUES ('$order_id','$id','$unitprice','$discount','$qty')") or die("".mysqli_error($db));
$newqty=$qty_available-$qty;
mysqli_query($db,"UPDATE products SET available_quantity='$newqty' WHERE product_id='$id'");
}
$k++;
}
}
$msg="checkedout_successfully";
//unset inserted order :
unset($_SESSION['cartProds']);
$_SESSION['nOfProds']=0;
echo $msg;
}
}
I need to stop the submission and to not insert anything when one of the quantities is equal to Zero.
What I am getting is that if 2 users want to checkout 3 same products at the same time (one product is added to order table of user 1 , and the other 2 are added to order table of user2 ..which is wrong , I want in this case to make checkout successful for only one user, and to give a warning alert msg for the second user)
Ajax code:
$(document).ready(function() {
$("#enableAddress").change(function(){
if($("#enableAddress").prop("checked") == true){
$("#addressCart").prop('disabled',false);
}
else{
$("#addressCart").prop('disabled',true);
}
});
$("#checkoutBtn").click(function(){
var ids_array = [];
$('input[name^=checkoutHidden]').each(function(){
ids_array.push($(this).val());
});
var qty_array = [];
$('input[name^=quantity]').each(function(){
qty_array.push($(this).val());
});
var telCart = $("#telCart").val();
var addressCart = $("#addressCart").val();
$.ajax({
type: "POST",
url: "../phpOnly/checkOut.php",
data: { ids_array:ids_array, qty_array:qty_array,addressCart:addressCart},
success: function(msg) {
// alert(msg);
if(msg=="checkedout_successfully"){
// $("#orderMsg").fadeIn(3000);
// $("#cartBody").fadeOut(2000);
// $("#nOfProdsSpan").text('0');
alert(msg);
}
else if(msg=="unable_checkout"){
// window.location.href="#";
// $("#inavailableMsg").fadeIn(1000);
alert(msg);
}
}
});
});
});
Don't forget that I am taking into consideration that the available quantity is only 1 ( which only one user can order it only)

Answer 1

An approach to handle concurency would be to change the logic, and start the process by attempting to update the products table, with a where clause that ensures that the requested quantity is available, like so:
UPDATE products
SET available_quantity = available_quantity - 1
WHERE
product_id = :id
AND product_active = 1
AND available_quantity > 0
You can then check the number of rows affected by the query:
if one row was affected, then it means the that check out did succeed; you can then do the rest of the process (create the order and the order details records)
if no row was affected, then the check out failed; do not create the order
With this technique, you let the database handle the concurrency for you, and you are guaranteed to never sell more product than available.