I use this and it works
header('Content-Description: File Transfer');
header('Content-Type: text/csv; charset=UTF-16LE');
header('Content-Disposition: attachment; filename=file.csv');
header('Content-Transfer-Encoding: binary');
header('Expires: 0');
header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
header('Pragma: public');
// output headers so that the file is downloaded rather than displayed
// create a file pointer connected to the output stream
$output = fopen('php://output', 'w');
fputs( $output, "\xEF\xBB\xBF" );
// output the column headings
fputcsv($output, array('Thông tin khách hàng dang ký'));
// fetch the data
$setutf8 = "SET NAMES utf8";
$q = $conn->query($setutf8);
$setutf8c = "SET character_set_results = 'utf8', character_set_client =
'utf8', character_set_connection = 'utf8', character_set_database = 'utf8',
character_set_server = 'utf8'";
$qc = $conn->query($setutf8c);
$setutf9 = "SET CHARACTER SET utf8";
$q1 = $conn->query($setutf9);
$setutf7 = "SET COLLATION_CONNECTION = 'utf8_general_ci'";
$q2 = $conn->query($setutf7);
$sql = "SELECT id, name, email FROM myguests";
$rows = $conn->query($sql);
$arr1= array();
if ($rows->num_rows > 0) {
// output data of each row
while($row = $rows->fetch_assoc()) {
$rcontent = " Name: " . $row["name"]. " - Email: " . $row["email"];
$arr1[]["title"] = $rcontent;
}
} else {
echo "0 results";
}
$conn->close();
// loop over the rows, outputting them
foreach($arr1 as $result1):
fputcsv($output, $result1);
endforeach;