// ---- previously --------
if ($user !== null) {
  $a = $user->address;

  if ($a !== null) {
    $b = $user->getBirthday();

    if ($b !== null) {
      $bday = $b->asString();
    }
  }
}

// ---- in PHP 8 --------
$bdate = $user?->address?->getBirthday()?->asString();