$i = 1;

$wynik = mysql_query("SELECT * FROM signals WHERE id_robot = $robot")
    or die('Błąd zapytania');

    if(mysql_num_rows($wynik) > 0) {
        while($r = mysql_fetch_array($wynik)) {

            $id_signal = $r['id_signal'];

            $signal = mysql_query("SELECT * FROM signals INNER JOIN data USING(id_signal) WHERE id_signal = $id_signal ORDER BY data.time ASC LIMIT 1 ");
            if(mysql_num_rows($signal) > 0) {
                while($sig = mysql_fetch_array($signal)) {
                    //echo 'Time: '.$sig['time'].' Value: '.$sig['value'].' ';

                    $data[$i]['time'] = $sig['time'];
                    $data[$i]['value'] = $sig['value'];



                    $id_sensor= $sig['id_sensor'];

                    $sensor = mysql_query("SELECT * FROM signals INNER JOIN sensors USING(id_sensor) WHERE id_sensor = $id_sensor");

                    if(mysql_num_rows($sensor) > 0) {
                        while($sen = mysql_fetch_array($sensor)) {
                            //echo 'Sensor: '.$sen['description'].' ';

                            $data[$i]['sensor'] = $sen['description'];

                        }
                        //echo 'Signal: '.$r['id_sensor'].' <br/>';
                    $data[$i]['signal'] = $r['id_sensor'];


                    }

                }

                $i++;
            }

        }

    }else {
        echo 'Nie ma danych';
    }

    print_r($data);

    echo json_encode($data);