SELECT * FROM (SELECT `id_reysa` FROM `Reys` WHERE `otkuda`=
(SELECT `id_aeroporta` FROM `Aeroporti` WHERE `gorod` = "Москва") and `kuda`=
(SELECT `id_aeroporta` FROM `Aeroporti` WHERE `gorod` = "Лос-Анджелес")) as reyses

$sql = "SELECT * FROM (SELECT `id_reysa` FROM `Reys` WHERE `otkuda`=(SELECT `id_aeroporta` FROM `Aeroporti` WHERE `gorod` = "".$search_from."")
and `kuda`=(SELECT `id_aeroporta` FROM `Aeroporti` WHERE `gorod` = "".$search_to."")) as reyses";

/*$sql = "SELECT * FROM `Reys`"; -этот запрос работает, выводит 2 строки*/

if(!$result = $mysqli->query($sql))
{
    echo "Запрос: " . $sql . "n";
    echo "Номер_ошибки: " . $mysqli->errno . "n";
    echo "Ошибка: " . $mysqli->error . "n";
    exit;
}

if($result->num_rows===0)
{
    echo $result->num_rows; //Выводит 0
    exit;
}

while ($row = $result->fetch_assoc())
{
    print_r($row);
    echo "<br />";
}