Why do I get an undefined method error in PHP?

I'm trying to call a PHP function in a separate PHP file from another PHP file. First is the shortened PHP file where I call the function, then there is the shortened PHP file with the method I'm trying to call.

require("./includes/themaDB.inc.php");
$themaDB = new ThemaDB($username, $passwd);
session_start();
$email = $_SESSION['e_mail'];
if(isset($_POST['titel']) && isset($_POST['kort']) && isset($_POST['lang'])){
    $themaDB->voegThemaToe($titel, $kort, $lang, $email);
}

The code above gets called everytime the page loads.

class ThemaDB{

    private $db;

    public function __construct($user="root", $pwd="") {
        $this->db = new PDO("mysql:host=localhost;dbname=cvranken",$user, $pwd);
    }

    public function getThemas(){
        $stmt = $this->db->prepare("select * from thema");
        $stmt->setFetchMode(PDO::FETCH_OBJ);
        $stmt->execute();

        $themas = array();

        while($obj = $stmt->fetch()){
            array_push($themas, $obj);
        }

        return $themas;
    }

    public function voegThemaToe($titel, $kort, $lang, $email){
        $stmt = $this->db->prepare("insert into thema (titel, korte_omschrijving, lange_omschrijving, voorgesteld_door_docent_PK) values(:titel, :kort, :lang, :email)");
        $stmt->bindParam(":titel", $titel);
        $stmt->bindParam(":kort", $kort);
        $stmt->bindParam(":lang", $lang);
        $stmt->bindParam(":email", $email);
        $stmt->execute();
    }

}

This throws following fatal error.

Fatal error: Call to undefined method ThemaDB::voegThemaToe() in /home/jail/home/cvranken/public_html/docentThema.php on line 47

Line 47 in the real code is the one where I call voegThemeToe in the top code.

Sorry that some parameters are in Dutch :)