php接收到其他文件的重定向输出

if(is_file($filename)){
    $extension = strtolower(pathinfo($filename, PATHINFO_EXTENSION));
        if (in_array($extension, array('png', 'gif', 'jpeg' , 'jpg'))) {
            header("Content-Type: image/".$extension);
        } else {
            if($extension == 'css')
                header("Content-Type: text/css");
            if($extension == 'js')
                header("Content-Type: application/x-javascript");
        }
        header("Cache-Control: private, max-age=".$browser_cache);
        header('Expires: '.gmdate('D, d M Y H:i:s', time()+$browser_cache).' GMT');
        header('Content-Length: '.filesize($filename));
        $length = filesize($filename);
        $result = readfile($filename);
        readfile($filename);
        exit();
}