You can do the same thing a bit more elegant like so:
kirby()->hook('panel.page.update', function($page) {
if($page->parent()->parution()->isNotEmpty()) {
$p = $page->parution()->toString();
$pp = $page->parent()->parution()->toString();
if($p !== $pp) {
try {
$page->update(array(
'parution' => $pp
));
} catch (Exception $e) {
echo $e->getMessage();
}
}
}
});