How to decreasing a quantity with notifier - php

Question

Hi Guys This is my newbie question...
I started learning php three days ago...
This is my problem...
for example
I have a quantity 50 when I subracted by 10 the value 50 became 40
and the notifier alert me that number is less than 50
but still subracted when I subract it again .....
Here is My codes
if(isset($_POST['submit'])){
$id = $_GET['id'];
$old = $_POST['quantity'];
$new = $_POST['quantity1'];
$total = $new - $old;
if($total < 50 ){
echo "<h1 style='color: red'><center>CRITICAL LEVEL</center></h1>";
echo "<script>window.open('inventory.php' , '_self');</script>";
}
else{
$uiqry = $mysqli->prepare("UPDATE table_inventory SET quantity = ? WHERE id = ?");
$uiqry->bind_param('ii', $total,$id);
$uiqry->execute();
$uiqry->close();
echo "<script>alert('Subracted');</script>";
echo "<script>window.open('inventory.php', '_self');</script>";
}
}
I want When I the value 50 and I subract it from 10 the value is now 40 and the alert notify me
and when i subract it again by 10 the value become 30 and the notify popup again
how to do this
thanks in advance..
Sorry for my tongue twister english.. :D

Answer 1

if(isset($_POST['submit'])){
$id = $_GET['id'];
$old = $_POST['quantity'];
$new = $_POST['quantity1'];
$total = $new - $old;
if($total < 50 ){
echo "<h1 style='color: red'><center>CRITICAL LEVEL</center></h1>";
echo "<script>window.open('inventory.php' , '_self');</script>";
}
$uiqry = $mysqli->prepare("UPDATE table_inventory SET quantity = ? WHERE id = ?");
$uiqry->bind_param('ii', $total,$id);
$uiqry->execute();
$uiqry->close();
echo "<script>alert('Subracted');</script>";
echo "<script>window.open('inventory.php', '_self');</script>";
}