$query_tmp=mysqli_query($con,"select * from product_tmp where product_id='$product_id' and user_id='$user_id' and branch_id='$id_sucursal'");
$count=mysqli_num_rows($query_tmp);
if ($count==0){
    $sql=mysqli_query($con,"INSERT into product_tmp
    ( product_id, qty, unit_price, user_id, discount, branch_id, cventa)
    values ('$product_id','$qty','$unit_price', '$user_id','$descuento','$id_sucursal','$cventa')");
} else {
    $rw=mysqli_fetch_array($query_tmp);
    $actual_qty=$rw['qty'];
    $new_qty=$actual_qty+$qty;
    $update=mysqli_query($con,"update product_tmp set qty='$new_qty' where product_id='$product_id' and user_id='$user_id' and branch_id='$id_sucursal'");
}

<?php
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "myDB";

// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);
// Check connection
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}

$sql = "select * from product_tmp where product_id='$product_id' and user_id='$user_id' and branch_id='$id_sucursal'";

$result = $conn->query($sql);

if ($result->num_rows > 0)
{
   $sql = "INSERT into product_tmp
( product_id, qty, unit_price, user_id, discount, branch_id, cventa)
values ('$product_id','$qty','$unit_price', '$user_id','$descuento','$id_sucursal','$cventa')";

} else {

  $actual_qty = 0;

   while($row = $result->fetch_assoc())
  {
     $actual_qty += $row['qty'];
  }

  $sql = "update product_tmp set qty='$actual_qty' where product_id='$product_id' and user_id='$user_id' and branch_id='$id_sucursal'";
}
?>