<?php

session_start();
include 'databaseFuncties.php';
if (productInWinkelwagen($id, $_POST[aantal])) {
    echo 'Succes!';
} else {
    echo 'Mislukt.';
}
<?php

include 'functies.php';
$aantal = $_POST['aantal'];
$id = $_POST['id'];
$klant_id = $_POST['account_id'];
if (productInWinkelwagen($id, $aantal, $klant_id)) {
    echo '<span class="good">Geplaatst</span>';
} else {
    echo '<span class="bad">Mislukt</span>';
}