How to append to a XML file with PHP preferably with SimpleXML
With SimpleXML, you can use the addChild() method.
$file = 'xml/config.xml';
$xml = simplexml_load_file($file);
$galleries = $xml->galleries;
$gallery = $galleries->addChild('gallery');
$gallery->addChild('name', 'a gallery');
$gallery->addChild('filepath', 'path/to/gallery');
$gallery->addChild('thumb', 'mythumb.jpg');
$xml->asXML($file);
Be aware that SimpleXML will not "format" the XML for you, however going from an unformatted SimpleXML representation to neatly indented XML is not a complicated step and is covered in lots of questions here.