edit XML with simpleXML
Sure you can edit with SimpleXML:
$input = <<<END
<?xml version='1.0' standalone='yes'?>
<documents>
<document>
<name>spec.doc</name>
</document>
</documents>
END;
$xml = new SimpleXMLElement($input);
$xml->document[0]->name = 'spec.pdf';
$output = $xml->asXML();
Take a look at the examples.
Load your XML with SimpleXML and make the changes. Then you can use the asXML method to save the XML to a file (you pass the filename as the argument):
$xml = new SimpleXMLElement( $xmlString );
// do the manipulation here
$xml->asXML ( '/path/to/your/file.xml' );