readfile下载文件添ob_clean作用

$file = '12.jpg';

if(file_exists($file)){
	
	header('Content-Description: File Transfer');
	header('Content-type: application/octet-stream');
	header('Content-Disposition: attachment; filename=' . basename($file));
	header('Content-Transfer-Encoding: binary');
	header('Expires: 0');
	header('Cache-control: must-revalidate');
	header('Pragma: public');
	header('Content-Length: ' . filesize($file));
	ob_clean();
	flush();
	readfile($file);
	exit();
}