function next_sibling()
{
if ($this->parent === null) {
return null;
}
$idx = 0;
$count = count($this->parent->children);
while ($idx < $count && $this !== $this->parent->children[$idx]) {
++$idx;
}
if (++$idx >= $count) {
return null;
}
return $this->parent->children[$idx];
}