$html = '
<ul>
<li>Foo</li>
<li>Bar</li>
<li>Baz</li>
</ul>
';
$document = new Document($html);
$list = $document->first('ul');
// string(3) "Baz"
echo '<br><b>$list->child(2)->text():</b><br>'.highlight_string(print_r($list->child(2)->text(), true), true).'<br>';
// string(3) "Foo"
echo '<br><b>$list->firstChild()->text():</b><br>'.highlight_string(print_r($list->firstChild()->text(), true), true).'<br>';
// string(3) "Baz" - нет ничего
echo '<br><b>$list->lastChild()->text():</b><br>'.highlight_string(print_r($list->lastChild()->text(), true), true).'<br>';
$document = new Document($html);
$item = $document->find('ul > li')[1];
echo '<br><b>$item->previousSibling():</b><br>'.highlight_string(print_r($item->previousSibling()->text(), true), true).'<br>';
echo '<br><b>$item->nextSibling():</b><br>'.highlight_string(print_r($item->nextSibling()->text(), true), true).'<br>';