未识别的索引和使用未识别的常量[重复]

Notice: Undefined index: qty in C:\xampp\htdocs\ecommerce\cart.php on line 130

Notice: Use of undefined constant qty - assumed 'qty' in C:\xampp\htdocs\ecommerce\cart.php on line 137

<?php
if(isset($_POST['update_cart'])){ //line 128

     $qty =$_POST['qty'];

     $update_qty ="update cart set qty='$qty'";
     $run_qty =mysqli_query($con, $update_qty);

     $_SESSION ['qty']=$qty;

     $total=$total*qty; //line 137
}
?>

</div>

you forgot $ before qty in line 137 and seems like $_POST['qty'] is empty

$total=$total*qty;

suppost to be

$total=$total*$qty;

fixed version:

if(isset($_POST['update_cart'])){
     $qty =$_POST['qty'];
     $update_qty ="update cart set qty='$qty'";
     $run_qty =mysqli_query($con, $update_qty);
     $_SESSION ['qty']=$qty;
    $total=$total*&qty;
 }

hope it helps

here is the proper way to concat strings. You are missing a $ and assigning a variable for qty is totaly useless.

if (isset($_POST['update_cart']) !==false) {
    $update_qty = 'update cart set qty="' . $_POST['qty'] . '"';
    $run_qty = mysqli_query($con, $update_qty);
    $_SESSION ['qty'] = $_POST['qty'];
    $total = ($total * floatval($_POST['qty']));
}