modification du constructeur :

function __construct( $libelle="", $type="")
    {
        $this->libelle = (string) $libelle;
        $this->type = (string) $type;
    }

remplacer par ==>

function __construct($id=0, $libelle="", $type="")
    {
        $this->id = (int) $id;
        $this->libelle = (string) $libelle;
        $this->type = (string) $type;
    }