/**
 * Filters a $tree on depth and parent. This allows you to
 * get, for example, all of the terms at a certain depth,
 * or all of the terms of a certain parent at a certain depth.
 *
 * @param $tree
 *   As in the result of taxonomy_get_tree()
 * @param $depth
 *   The integer value of the depth. $tree will be filtered to 
 *   only contain terms with this depth ($term->depth). 
 * @param $parent
 *   A tid. $tree will be filtered to only contain the children of 
 *   this tid.
 */ 
function taxonomy_filter_tree(&$tree, $depth = NULL, $parent = NULL) {
  if ($count = count($tree)) {
    if (!is_null($depth)) {
      $tree = array_values(array_filter(array_map('taxonomy_filter_on_depth', $tree, array_fill(0, $count, $depth)), 'taxonomy_filter_not_null'));
    }
  }
  
  if ($count = count($tree)) {
  	if (!is_null($parent)) {
  		$tree = array_values(array_filter(array_map('taxonomy_filter_on_parent', $tree, array_fill(0, $count, $parent)), 'taxonomy_filter_not_null'));
  	}
  }
}

/**
 * Utility function used by taxonomy_filter_tree
 */
function taxonomy_filter_on_depth($term, $depth) {
	return ($term->depth == $depth) ? $term : NULL;
}

/**
 * Utility function used by taxonomy_filter_tree
 */
function taxonomy_filter_on_parent($term, $parent) {
	return in_array($parent, $term->parents) ? $term : NULL;
}

/**
 * Utility function used by taxonomy_filter_tree
 */
function taxonomy_filter_not_null($term) {
	return !is_null($term);
}