How to update xml file using PHP?

I have xml file like this --> examples.xml

<?xml version="1.0" encoding="UTF-8"?>
<inventors>
 <person>
 <name>anie</name>
 <comment>good</comment>
 </person>
</inventors>

This is my php file --> index.php

<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);
 ?>

 <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>

 <?php


 if (isset($_POST['submit'])) 
 { 
$name->nodeValue = $_POST['namanya'];
$comment->nodeValue = $_POST['commentnya'];
htmlentities($xml->save('examples.xml'));

 }


 ?>

it cant be work if i click button submit, on browser view have 2 result input. if i click button submit, i want result is new input.