Fatal error: Call to a member function query() on null in /var/www/html/forum/kernel/model.class on line 40

class connex {
public static $bdd;
const USERNAME="root";
const PASSWORD="root";
const HOST="localhost";
const DB="forum";
public function connexion() {
     try {
        $username = self::USERNAME;
        $password = self::PASSWORD;
        $host = self::HOST;
        $db = self::DB;
        $bdd = new PDO("mysql:dbname=$db;host=$host", $username, $password);

      }catch (PDOException $e){
            print "Erreur, " . $e->getMessage() . "<br />";
            die();
         }
    return true;
}

public function deconnexion() {
    $bdd = null;
    return true;
    }