I've done this recently and it worked quite well:

  // Turn our XML string into a nested, associative array.
  $data = new SimpleXMLElement($request->data);
  $data = drupal_json_decode(drupal_json_encode($data));

Just remember that you want to use empty() instead of != FALSE to check for "emptiness" as things that are empty strings in your XML turn into empty arrays after conversion.

$request in that snippet comes from drupal_http_request() so yeah, $request->data is the XML string.

The encoding then decoding is just tricking PHP into turning it into an array for you by encoding it as a JSON string first. Probably not the highest performance way to do it, but it is likely the easiest to code.

I like QueryPath - http://drupal.org/project/querypath

I use it for a variety of purposes including XML and find that makes my life pretty easy.

Thank you very much!!! :)

<?php
// this code is to parse xml file stored inside a node->field and called from rules
// funtion from my rules action
function procesarfactura_rules_action_info() {
  $actions = array(
    'procesar_factura_xml' => array(
        'label' => t('Procesar factura'),
        'group' => t('factura'),
        'parameter' => array(
            'node' => array(
                'type' => 'node',
                'label' => t('Nodo a procesar'),
                ),
            'file' => array(
                    'type' => 'file',
                    'label' => t('Archivo a procesar'),
                    ),
        ),
      ),
    );
      return $actions;
}

// code to load the file and parse xml content
function procesar_factura_xml($file);
$uri = $file->field_factura_xml['und'][0]['uri'];
$files = file_load_multiple(array(), array('uri' => $uri));
$file = reset($files); // If empty, $file will be false, otherwise will contain the required file
$xml = simplexml_load_file($uri);

// now you have a xml string to parse from variable $xml.
// here goes code to parse a xml string.

?>