public function voerIntrestDoor()
{
parent::stort(parent::getSaldo() * self::$intrest);
}
<?php
require_once './Rekening.php';
?>
<!DOCTYPE HTML>
<html>
<head>
<meta charset=utf-8>
<title>Rekeningnummers</title>
</head>
<body>
<h1>
<?php
$rek = new Rekening("091-0122401-16");
echo "Het saldo is: " . $rek->getSaldo() . "<br />";
$rek->stort(200);
echo "Het saldo is: " . $rek->getSaldo() . "<br />";
$rek->voerIntrestDoor();
echo "Het saldo is: " . $rek->getSaldo() . "<br />";
?>
</h1>
</body>
</html>