Your program shows the form firstly, then saves data secondly. So when you click button submit, the program shows the form with previous data first and then, saves 'new' data.
Therefore following code works well.
<script src="http://code.jquery.com/jquery-latest.min.js"></script>
<?php
$xml = new DOMDocument('1.0', 'utf-8');
$xml->formatOutput = true;
$xml->preserveWhiteSpace = false;
$xml->load('examples.xml');
//Get item Element
$element = $xml->getElementsByTagName('person')->item(0);
//Load child elements
$name = $element->getElementsByTagName('name')->item(0);
$comment = $element->getElementsByTagName('comment')->item(0) ;
//Replace old elements with new
$element->replaceChild($name, $name);
$element->replaceChild($comment, $comment);
?>
<?php
if (isset($_POST['submit']))
{
$name->nodeValue = $_POST['namanya'];
$comment->nodeValue = $_POST['commentnya'];
htmlentities($xml->save('examples.xml'));
}
?>
<form method="POST" action=''>
name <input type="text-name" value="<?php echo $name->nodeValue ?>" name="namanya" />
comment <input type="text-comment" value="<?php echo $comment->nodeValue ?>" name="commentnya"/>
<input name="submit" type="submit" />
</form>