php运用readfile函数给隐藏下载文件地址增加权限判

<?php   
$file = get_file_address();// 文件的真实地址（支持url,不过不建议用url）   
  
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, post-check=0, pre-check=0');   
    header('Pragma: public');   
    header('Content-Length: ' . filesize($file));   
    ob_clean();   
    flush();   
    readfile($file);   
    exit;   
}   
?>