Why does asXML() sometimes not work in simplexml?

Question

PHP

Simplexml_load_file('some.xml')->asXML(); works weird for me.
In some cases, everything works perfectly, and in some it displays an error 500.
That is, everything works in this code:

<?php
  $login = $_POST['user'];
  $xml = simplexml_load_file('online.xml');
  $xml->$login->lastactiv=date("YmdHi");
  $xml->$login->nlastactiv=date("Y.m.d H:i");
    file_put_contents('online.xml', $xml->asXML());
?>

And this is the 500 error:

<?php
  $title = $_POST['title'];
  $email = $_POST['email'];
  $name = $_POST['name'];
  $tz = $_POST['timezone'];
  $a = $_POST['about'];
  $sms = $_POST['sendSMS'];
  $st = $_POST['sendsTo'];

  $xml = simplexml_load_file('some.xml');
  $xml->$login->TimeZone=$tz;
  $xml->$login->about=$a;
    $xml->$login->sendSMS=$sms;
    $xml->$login->sendsTo=$st;
    file_put_contents('some.xml', $xml->asXML());
?>

And I'm sure that the error is in $ xml- >asXML(), because when I don't use it, everything is fine, but when I write echo $xml->asXML();, that's it, error 500

validations found no errors.
Thank you.