> I've tried simplexml and a few third-party scripts, one that loads the 
> xml into an associative array rather than an object, and nothing lets me 
> get at 'red' or 'blue'. 

I needed something similar for my module. First of all, I wanted it to be compatible with PHP4. This means I needed to use xml_parser functions. Second, I didn't want the user to have to install another module or library.

I ended up using two functions (one is recursive):

/**
 * Takes an XML string, and returns a multidimensional array. Each "dimension"
 * may be a text string, a key-value array of tag attributes, and/or another
 * multidimensional array containing child nodes.
 *
 * @param $string
 *     String containing XML data to parse
 * @return
 *     Multidimensional array with attributes, values, and children
 */
function _discogs_xml_to_array(&$string) {
  $parser = xml_parser_create();
  xml_parser_set_option($parser, XML_OPTION_CASE_FOLDING, 0);
  xml_parse_into_struct($parser, $string, $vals, $index);
  xml_parser_free($parser);

  $multi_array = array();
  foreach ($vals as $val) {
    $tag = $val['tag'];
    if ($val['type'] == 'open') {
      if (isset($multi_array[$tag])) {
        if (isset($multi_array[$tag][0])) {
          $multi_array[$tag][]=array();
        }
        else {
          $multi_array[$tag]=array($multi_array[$tag], array());
        }
        $cv = &$multi_array[$tag][count($multi_array[$tag])-1];
      }
      else {
        $cv = &$multi_array[$tag];
      }
      if (isset($val['attributes'])) {
        foreach ($val['attributes'] as $k => $v) {
          $cv['_attributes'][$k] = $v;
        }
      }
      $cv['_nodes'] = array();
      $cv['_nodes']['_recursion'] = &$multi_array;
      $multi_array = &$cv['_nodes'];
    }
    elseif ($val['type'] == 'complete') {
      if (isset($multi_array[$tag])) {
        if (isset($multi_array[$tag][0])) {
          $multi_array[$tag][] = array();
        }
        else {
          $multi_array[$tag] = array($multi_array[$tag], array());
        }
        $cv = &$multi_array[$tag][count($multi_array[$tag])-1];
      }
      else {
        $cv = &$multi_array[$tag];
      }
      if (isset($val['attributes'])) {
        foreach ($val['attributes'] as $k => $v) {
          $cv['_attributes'][$k] = $v;
        }
      }
      if (isset($val['value'])) {
        $cv['value'] = trim($val['value']);
      }
    }
    elseif ($val['type'] == 'close') {
      $multi_array = &$multi_array['_recursion'];
    }
  }
  // Clean up the recursion and merge attributes with child nodes
  $multi_array = _discogs_xml_clean($multi_array);

  return $multi_array;
}

/**
 * Helper function for _discogs_xml_to_array.
 *
 * Cleans up the array structure, by getting rid of the recursive data, and
 * merging the values, attributes and child nodes.
 *
 * @param array $old_array
 *   Old array to clean up
 * @return array $new_array
 *   Cleaned up array
 */
function _discogs_xml_clean($old_array) {
  // Double check that it's an array, for recursion
  if (!is_array($old_array)) {
    return $old_array;
  }
  foreach ($old_array as $k => $v) {
    if ($k === '_recursion') {
      unset($old_array[$k]);
    }
    elseif (is_array($old_array[$k]['_nodes']) && is_array($old_array[$k]['_attributes'])) {
      $old_nodes = _discogs_xml_clean($old_array[$k]['_nodes']);
      $new_merged = array_merge($old_array[$k]['_attributes'], $old_nodes);
      $new_array[$k] = $new_merged;
    }
    elseif (isset($old_array[$k]['value']) && is_array($old_array[$k]['_attributes'])) {
      $value['value'] = trim($old_array[$k]['value']);
      $new_merged = array_merge($old_array[$k]['_attributes'], $value);
      $new_array[$k] = $new_merged;
    }
    elseif (is_array($old_array[$k]['_nodes'])) {
      $new_array[$k] = _discogs_xml_clean($old_array[$k]['_nodes']);
    }
    elseif (isset($old_array[$k]['_attributes'])) {
      $new_array[$k] = $old_array[$k]['_attributes'];
    }
    elseif (isset($old_array[$k]['value'])) {
      $new_array[$k] = $old_array[$k]['value'];
    }
    else {
      $old_nodes = _discogs_xml_clean($old_array[$k]);
      $new_array[$k] = $old_nodes;
    }
  }
  return $new_array;
}

So, if you passed your XML file to the first function, this would be the resulting multidimensional array:

Array
(
  [this] => Array
    (
      [that] => Array
        (
          [0] => Array
            (
              [parm1] => a
              [parm2] => b
              [parm3] => c
              [value] => red
            )

          [1] => Array
            (
              [parm1] => e
              [parm2] => f
              [parm3] => d
              [value] => blue
            )
        )
    )
)

It's complicated, but it works.

-Karl.