readdir（）：33不是有效的目录资源

I know we have duplicates of this but still not getting exact solution. Here is my code

if ($lib_dir = @opendir($_SERVER["DOCUMENT_ROOT"]."/lib")) {
    while (false !== ($lib_file = readdir($lib_dir))) {
        if ((preg_match("/\.php$/", $lib_file)) & ($lib_file!="loader.php"))     
        {
            include_once($_SERVER["DOCUMENT_ROOT"]."/lib/$lib_file");
        }
    }
    closedir($lib_dir);

}

Thanx in advance