想问问这段代码为啥不成功

<?php	session_start();	$user = $_SESSION['login_name'];	$filename = $_GET["f"];	$file = "../upload/".$user."/".$filename;	if (file_exists($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;	}?>