Codice PHP:
 <?php 
          
            $sql="SELECT * FROM products WHERE id_product IN ("; 
                      
                    foreach($_SESSION['cart'] as $id => $value) { 
                        $sql.=$id.","; 
                    } 
                      
                    $sql=substr($sql, 0, -1).") ORDER BY name ASC"; 
                    $query=mysqli_query($link,$sql);
                    $totalprice=0; 
                    while($row=mysqli_fetch_array($query)){ 
                        $subtotal=$_SESSION['cart'][$row['id_product']]['quantity']*$row['price']; 
                        $totalprice+=$subtotal;
                        
                        $quantita_r=$row['Quantita_rimaste'] - ($_SESSION['cart'][$row['id_product']]['quantity']);
                        $quantita_v=$row['Quantita_vendute'] + ($_SESSION['cart'][$row['id_product']]['quantity']);
                        
                    ?> 
                        <tr> 
                            <td><?php echo $row['name'] ?></td> 
                            <td><input type="text" name="quantity[<?php echo $row['id_product'] ?>]" size="5" value="<?php echo $_SESSION['cart'][$row['id_product']]['quantity'] ?>" /></td> 
                            <td><center><?php echo $row['price'] ?> €</center></td> 
                            <td><center><?php echo $_SESSION['cart'][$row['id_product']]['quantity']*$row['price'] ?> €</center></td>
                            <td><center><?php echo $quantita_r ?></center></td>
                            <td><center><?php echo $quantita_v ?></center></td>
                        </tr> 
                    <?php 
                    
                    } 
        ?> 
                    <tr> 
                        <td colspan="6">Prezzo totale di vendita: <?php echo $totalprice ?> €</td> 
                    </tr> 
          
    </table>
    <br /> 
    <button type="submit" name="submit" class="entra">Aggiorna carrello</button>
    </form><br>
    
    <form method="POST">
    <input type="submit" name="conferma" value="Conferma Vendita" class="entra"/>
    </form>
    


<?php
 if(isset($_POST['conferma'])){
     
    
 $sql_v = "UPDATE products SET 
     Quantita_rimaste={$quantita_r}, 
     Quantita_vendute={$quantita_v} 
     WHERE id_product='$id'";  
     $query=mysqli_query($link,$sql_v);
     
 } 
?>
risultato:



clicco su conferma vendita e ottengo:



le quantità non vengono aggiornate bene