XML DOM object wierd problem

Hello, I have an XML file, for example:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<flat-profile xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">

<SomeElement>ccccc</SomeElement>

</flat-profile>

and I can easily edit it with this simple code:

<?php

$dom = new DOMDocument;
$dom->load('XML/sample.xml');

$SomeElement = $dom->getElementsByTagname('SomeElement')->item(0)->firstChild->nodeValue;
echo '<br> Current value is: ' . $SomeElement . '<br>';

$dom->getElementsByTagname('SomeElement')->item(0)->firstChild->nodeValue = 'aaaaa';
$SomeElementValue = $dom->getElementsByTagname('SomeElement')->item(0)->firstChild->nodeValue;
echo '<br> New value is: ' . $SomeElementValue . '<br>';

$dom->save('XML/sample.xml');

?>

This works fine, but when I, in XML file, instead of:
<SomeElement>ccccc</SomeElement>

have:
<SomeElement/>
or
<SomeElement></SomeElement>

It want work, it does not want to change SomeElement to a value 'aaaaa'.

Is there some mistake with this code:
$dom->getElementsByTagname('SomeElement')->item(0)->firstChild->nodeValue;

or ?????

Please help...
Thanks