Its blank because you are writing to file. you should write to output using php://output instead and also send header information to indicate that it's csv.
Example
header('Content-Type: text/csv');
header('Content-Disposition: attachment; filename="sample.csv"');
$data = array(
'aaa,bbb,ccc,dddd',
'123,456,789',
'"aaa","bbb"'
);
$fp = fopen('php://output', 'wb');
foreach ( $data as $line ) {
$val = explode(",", $line);
fputcsv($fp, $val);
}
fclose($fp);