function VoorraadProduct(){

    $conn = Connect();

    $sql = "SELECT vooraad FROM product WHERE naam='Magnoli'";
    $result = $conn->query($sql);
    $result = $result->fetch(PDO::FETCH_ASSOC);

    foreach ($result as $row) {
        $som = $row - 1;

        $sql = "UPDATE product SET vooraad=$som WHERE naam='Magnoli'";
        $conn->query($sql);
    }
}
if(isset($_POST['submit'])) {
    VoorraadProduct();
}

function ToonProducten($rows){
    if ($rows > 0) {
        // output data of each row
        for ($i = 0; $i < count($rows); $i++) {
            $row = $rows[$i];
            echo '<div class="card">';
            echo "<img src=" . $row['url_afbeelding'] . " width=200px height=100px>";
            echo "<br>";
            echo "Naam: " .   $row['naam'] . "<br>";
            echo "Prijs: " .   $row['prijs'] . "<br>";
            echo "Vooraad: " .   $row['vooraad'] . "<br>";
            echo "Categorieen: " .   $row['categorieen'] . "<br>";
            echo "<br>";
            echo "<form method=post>";
            echo "<input type=submit name=submit value='Plaats in winkelwagen'>";
            echo "</form>";

            echo "</div>";
        }
    } else {
        echo "0 results";
    }
}