Problema Clase con extends

[Yoi]

ok, mira, mas o menos es asi es como tengo las clases:

class variables{
var $hostname_connection = "localhost";
var $database_connection = "clustnet_ecommerce";
var $username_connection = "root";
var $password_connection = "";
}

class clasepro extends variables{
private $conn;
function nn(){
$this->conn= (mysql_connect($this->hostname_connection,$this->username_connection,$this->password_connection)) or die(mysql_error());
mysql_select_db($this->database_connection,$this->conexion );

}

public function pregunta($consulta){
$resultado = mysql_query($consulta,$this->conn);
return $resultado;
}
public function asociado($consulta){
return mysql_fetch_array($consulta);
}

}

class cart extends clasepro{
function impri(){


$qur = "SELECT *FROM tablaWHERE id=" . $cualquiera;

$rst = $this->pregunta($qur);
$row = $this->asociado($rst);
$foto = $row['imagen'];

}
}

mira, mas o menos la tengo asi: el error sale en las siguientes lineas:
$rst = $this->pregunta($qur);
$row = $this->asociado($rst);

Warning: mysql_query() expects parameter 2 to be resource, null given in sitio\conectar.php on line 33

Warning: mysql_fetch_assoc() expects parameter 1 to be resource, null given in sitio/conectar.php on line 4