$xml = new DOMDocument();
$xml->load('myhappyfarmsave.xml');
$xpath = new DOMXPath($xml);

$elements = $xpath->query('/myhappyfarmsave/platform');

$domElement = $elements->item(0);
var_dump($domElement->textContent);
$domElement->textContent = "newData";

$xml->save('myhappyfarmsavenew.xml');

<?xml version="1.0" encoding="UTF-8"?>
<myhappyfarmsave version="3">
    <platform>Bazaar</platform>
    <timestamp>1356876534</timestamp>
    <lastlogtime>1356823542</lastlogtime>
</myhappyfarmsave>

$domElement = $elements->item(0);
$domElement->nodeValue = "newData";

$xml->save('myhappyfarmsavenew.xml');