ok risolto in questo modo:

codice:
$data = [];
    $markets = [];
    if(!$query || mysqli_num_rows($query) == 0){
        $data = ["esito" => "ko"];
    } else {
        
        $fields = [];
        while($row = mysqli_fetch_row($query)){
        $fields = [
                "ID" => $row[0],
                "lat" => $row[1],
                "lng" => $row[2]
            ];
            array_push($markets, $fields);
        }
        $data = [
            "esito" => "OK",
            "markets" => $markets
        ];
    }