ผมใช้ force download แล้วมันโหลดไม่ขึ้น ไฟล์มันมาแค่ 0 bytes เองครับ ช่วยผมที

ob_start();
date_default_timezone_set("Asia/Bangkok");
$dbhost = '127.0.0.1';
$dbuser = 'root';
$dbpass = '1234';
$conn = mysql_connect($dbhost, $dbuser, $dbpass);
if(! $conn )
{
  die('Could not connect: ' . mysql_error());
}
$table_name = "detail";
$backup_file  = "detail.sql";
$sql = "SELECT * INTO OUTFILE '$backup_file' FROM $table_name";

mysql_select_db('aaa');
$retval = mysql_query( $sql, $conn );

// send dump file to the output
header('Content-Description: File Transfer');
header('Content-Type: application/octet-stream');
header('Content-Disposition: attachment; filename='.basename($table_name . "_" .date("Y-m-d_H-i-s").".sql"));
flush();
echo $dump;
exit();