Fouten in classes

<?php

class KosLib
{
	function __construct()
	{
           if(!class_exists('Database')){
               include_once('/var/www/KosLib/config/database.php');
               $this->db = new Database();
           }
           if(!class_exists('Kenteken')){
             include_once('/var/www/KosLib/classes/kenteken.php');
                $this->kenteken = new Kenteken();
           }
        }
        
        
}

<?php

class Kenteken extends KosLib
{
    function __construct() 
    {
        parent::__construct();
    }
    
    function getPlateTypes()
    {
        $sql = 'SELECT
                    ordernr,
                    description
                FROM
                    kentekens';
        
        return $this->db->query($sql);
    }
    
}
?>

<?php
ini_set('display_errors', 1);

    include('/var/www/KosLib/bootstrap.php');
   
    $KL = new KosLib;
    
    foreach($KL->kenteken()->getPlateTypes() as $plateType)
    {
        echo $plateType->description.'<br />';
    }

?>

Fatal error: Call to undefined method KosLib::kenteken() in /var/www/Testhoek/index.php on line 8