First of all, your XML doesn't look correct. The first <browser> should be <browsers> instead. But, this will work:
$xml = '<statbox title="demo2">
<browser>
<browser name="Mozzila">34</browser>
<browser name="Microsoft Internet Explorer 6">15</browser>
</browser>
</statbox>';
// Load XML string into DOM
// Use load() to load from a file
$xml_dom = DOMDocument::loadXML($xml);
// Get the first browser "parent"
$browser = $xml_dom->getElementsByTagName('browser')->item(0);
// Create new element, "line"
$firefox = $xml_dom->createElement('browser', '15'); // First make the <browser>15</browser>
$firefox->setAttribute('name', 'firefox'); // Add attribute name="firefox"
// Insert the new element, "line"
$browser->appendChild($firefox);
// Vola, new line added
print($xml_dom->saveXML());