Winkelwagen Gegevens Naar Database Verzenden

<?php
include ("includes/connect.php");
    if(isset($_POST['submit'])){
        foreach($_POST as $key => $value){
            $key = explode("-",$key);
            $key = end($key);
            $key = explode("submit",$key);
            $key = end($key);
            
            if(isset($_POST['quantity-'.$key]) <= 0){
                unset($_SESSION['cart'][$key]);
            } else if($_POST['quantity-'.$key] >= 50){
                $_SESSION['cart'][$key]['quantity'] = 50;
            } else {
                $_SESSION['cart'][$key]['quantity'] = $value;
            }
        }
    } error_reporting(0);
?>
<h1>Bekijk winkelwagen</h1>
<a href="beveiligdepagina.php?page=products">Ga terug naar de producten pagina.</a><br />
<?php $sql = "SELECT * FROM producten WHERE id_product IN (";
                foreach ($_SESSION['cart'] as $id => $value){
                    $sql .= $id . ",";
                }
                $sql = substr($sql,0,-1).") ORDER BY id_product ASC";
                $query = mysql_query($sql);
            if(empty($query)){
                echo "Je moet een product toevoegen om toegang te krijgen op deze pagina.<br /><br />";
            }
?>
<form method="post" action="#">
<fieldset>
    <table>
        <tr>
            <th>Naam</th>
            <th>Hoeveelheid</th>
            <th>Prijs per product</th>
            <th>Totaal prijs</th>
        </tr>
        <?php 
        
            $sql = "SELECT * FROM producten WHERE id_product IN (";
                foreach ($_SESSION['cart'] as $id => $value){
                    $sql .= $id . ",";
                }
                $sql = substr($sql,0,-1).") ORDER BY id_product ASC";
                $query = mysql_query($sql);
                $total_price = 0;
                if(!empty($query)){
                while ($row = mysql_fetch_array($query)){
                    $subtotal = $_SESSION['cart'][$row['id_product']]['quantity']*$row['prijs'];
                    $total_price += $subtotal;
                    ?>
                        <tr>
                            <td><?php echo $row['naam'];?></td>
                            <td><input type="text" name="quantity-<?php echo $row['id_product'];?>" size="5" value="<?php echo $_SESSION['cart'][$row['id_product']]['quantity'];?>" style="width: 30px; height: 18px; margin-left: 17px;"/></td>
                            <td><?php echo "€" . $row['prijs'];?></td>
                            <td><?php echo"€" .  $_SESSION['cart'][$row['id_product']]['quantity']*$row['prijs'];?></td>
                        </tr>
                     <?php
                } }
            ?>
            <tr>
                <td></td>
                <td></td>
                <td>Totaal prijs:</td>
                <td><?php echo"€" . $total_price;?></td>
             </tr>
          </table>
          <br />
          <input type="submit" name="submit" value="Update winkelwagen"/></fieldset>
       </form><br />
       <form method="POST" action="afrekenen.php">
       <input type="submit" name="submit" value="Afrekenen"/></fieldset>
       </form><br />
       <p>Om een product te verwijderen, dient u de hoeveelheid op <strong>0</strong> te zetten.</p>