How to get the menu tree in Drupal 7? Expanded or not.

page.tpl.php

<?php print $custom_main_menu; ?>

template.php

<?php
/**
 * Gets the data structure representing a named menu tree.
 *
 * Since this can be the full tree including hidden items, the data returned
 * may be used for generating an an admin interface or a select.
 *
 * @param $menu_name
 *   The named menu links to return
 * @param $max_depth
 *   Optional maximum depth of links to retrieve. Typically useful if only one
 *   or two levels of a sub tree are needed in conjunction with a non-NULL
 *   $link, in which case $max_depth should be greater than $link['depth'].
 * @param $attributes
 *   An associative array of key-value pairs to be converted to attributes of
 *   the menu wrapper.
 *
 * @return
 *   Returns HTML.
 */
function _MYTHEME_get_menu($menu_name, $max_depth = NULL, $attributes = array()) {
  $menu_output = &drupal_static(__FUNCTION__, array());

  if (!isset($menu_output[$menu_name])) {
    $tree = menu_tree_page_data($menu_name, $max_depth);
    $menu_output[$menu_name] = menu_tree_output($tree);
  }

  $attributes['role'][] = 'navigation';
  $attributes['class'][] = 'p6-' . $menu_name;
  return '<nav' . drupal_attributes($attributes) . '>' . render($menu_output[$menu_name]) . '</nav>';
}

/**
 * Preprocess variables for page.tpl.php
 */
function MYTHEME_preprocess_page(&$vars) {
  $is_front_page = drupal_is_front_page();
  $main_menu_name = variable_get('menu_main_links_source', 'main-menu');

  if ($is_front_page) {
    $vars['custom_main_menu'] = _MYTHEME_get_menu($main_menu_name, 1);
  }
  else {
    $vars['custom_main_menu'] = _MYTHEME_get_menu($main_menu_name, NULL);
  }
}