Drupal: Save additional path aliases for subpages. Could be used with nodes to make node/123/edit (with alias "foo") look like foo/edit. Prettier.

path_save_subpage_alias.php

/**
 * Implements hook_node_presave().
 */
function custom_node_presave($node) {
  // Get the source of the "base" path which has subpages.
  $source = 'node/' . $node->nid;

  // If the node's alias is "foo", foo/edit will be an alias for node/123/edit.
  custom_save_subpage_alias($source, 'edit');
  
  // If the node's alias is "foo", foo/debug will be an alias for node/123/devel.
  custom_save_subpage_alias($source, 'devel', 'debug');
}

function custom_save_subpage_alias($base_source, $source_suffix, $alias_suffix = NULL) {
  $base_alias = drupal_lookup_path('alias', $base_source);
  if ($base_alias) {
    // If no alias suffix, the source's suffix will be used for the alias too.
    if (empty($alias_suffix)) {
      $alias_suffix = $source_suffix;
    }

  $path = array(
      'source' => "$base_source/$source_suffix",
      'alias' => "$base_alias/$alias_suffix",
    );

    // Only save the subpage alias if it doesn't already exist.
    $alias_exists = (bool) drupal_lookup_path('source', $path['alias']);
    if (!$alias_exists) {
      path_save($path);
    }
  }
}