The stock was out of limit, still insert into order table - php

Question

$prodqty = mysql_query("SELECT quan FROM pro_list WHERE auto_id = $pid"); //get the current product quantity
if (mysql_num_rows($prodqty) != 0)
{
$row = mysql_fetch_array($prodqty);
$productqty = $row['quan'];
}
$nqty = $productqty-$q; //current product quantity minus order quantity to get new product quantity
if ($nqty >= 0)
{
$query2="UPDATE pro_list SET quan = $nqty WHERE auto_id = $pid"; //update the quantity in the product table
$result = mysql_query($query2);
if ($result)
echo "Successfully ";
else
echo "Unsuccesfully";
}
else
echo "Limit of quantity! .";
}
die('Thank You For Shopping With i-Supply System! your order has been sent to Admin.!');
}
the stock was out of limits, but the orders still insert into database. the order should not insert into table order because it already out of limit the quantity of products. Customer should make an order again. Why this should happen?

Answer 1

Your approach is fundamentally flawed. You will get a race condition if two or more updates are attempted at the same time. You should perform the check and update as a single query like this:
$query="UPDATE pro_list SET quan=quan-$q WHERE auto_id=$pid and quan>=$q";
$result = mysql_query($query) or die(mysql_error());
if (mysql_affected_rows() == 0) {
echo "Out of stock!";
}
In any case, you shouldn't be using mysql - it's deprecated. Use mysqli or PDO instead.