Get current viewed node if available

current_viewed_node.php

<?php
/**
* Used to get the current viewed node (works when viewed in page mode).
* @param array $node_types[optional] A filter on the type of node you want to see.
* @return object The node or null if not successfull.
*/
function helper_get_current_node($node_types = array()) {
  // Store the current node id, to avoid doing the URL testing
  // on every call to this function. I didn't store the node itself
  // because I was afraid of data changes during page processing.
  // Normally node_load() already does some static caching and I think
  // it handles cache updates correctly.
  static $nid;

  if (!isset($nid)) {
    $arg = arg(); // Get URL splitted.

    // What type of URL is it?
    switch ($arg[0]) {
      // Viewing a node or a revision of a node :
      case 'node':
        // If the node id is missing, null or not numeric
        if (!isset($arg[1]) || is_null($arg[1]) || !is_numeric($arg[1])) {
          $nid = false;
        }
        // Look at the 3rd part of the URL ('edit', 'view', 'revisions', ...)
        if (isset($arg[2])) {
          switch ($arg[2]) {
            case 'view':
              break;
            case 'revisions':
              // If we are not viewing a revision
              if (!isset($arg[4]) || $arg[4] != 'view') {
                $nid = false;
              }
              break;
            default: // 'edit', 'delete', etc...
              $nid = false;
          }
        }
        // If $nid has not been set, it means we where viewing a node.
        if (!isset($nid)) {
          $nid = $arg[1];
        }
        break;

      // Commenting a node :
      case 'comment':
        // If the URL just has /comment, or if the node id is missing or not numeric
        if (!isset($arg[1]) || !isset($arg[2]) || !is_numeric($arg[2])) {
          $nid = false;
        }
        // If $nid has not been set to false, it means we should be commenting a node.
        if (!isset($nid)) {
          $nid = $arg[2];
        }
        break;

      // URL doesn't start with something relative to node viewing
      default:
        $nid = false;
    }
   
  } // end if $nid is not set.

  // Return null if we are not viewing a node.
  if (!$nid) return null;

  // Load the node.
  $viewedNode = node_load($nid);

  // return null, if node not loaded, if node isn't the desired type
  // or if the user isn't allowed to see this node.
  if (!$viewedNode ||
      !node_access('view', $viewedNode) ||
      (count($node_types) > 0 && array_search($viewedNode->type, $node_types) === false)) {
    return null;
  }

  return $viewedNode;
}
?>

Pasted from http://drupal.org/node/160921#comment-2938906 for continence.