i complete my code on windows xampp to download text(or other file) file and its work good but when i upload the code on ubuntu server its does not work can you help me plz

$file = "$username.rsc";
$txt = fopen($file, "w") or die("Unable to open file!");
fwrite($txt,
"{
anything
}

");
fclose($txt);

header('Content-Description: File Transfer');
header('Content-Disposition: attachment; filename='.basename($file));
header('Expires: 0');
header('Cache-Control: must-revalidate');
header('Pragma: public');
  ob_clean();
    flush();
header('Content-Length: ' . filesize($file));
header("Content-Type: text/plain");
readfile($file);

the response on ubuntu is Unable to open file!