Drupal - How do I get the node ID from the URL?

I am not really sure what the difference between them is

• menu_get_object() returns the object associated with the page currently shown. If the code is executed when Drupal is showing the page example.com/node/1, then menu_get_object() will return the node object for the node whose ID is 1; if the page being shown is example.com/user/1, then menu_get_object('user') will return the user object for the user whose ID is 1.
• menu_get_item() returns an array containing an index for each field contained in the "menu_router" table. This includes:
  • path: the current path of the menu callback
  • load_functions: an array of function names (like node_load) to be called to load an object corresponding to a part of the current path
  • access_callback: the callback that checks if the currently logged in user has access to the menu item
  • page_callback: the name of the function that renders the page
  • title: the title for the page associated to the menu item
• drupal_lookup_path() is the function that returns you the internal path associated with the alias passed as argument. If "homepage" is the alias associated to example.com/node/1, then drupal_lookup_path("homepage") will return "node/1". Path aliases can be associated to any internal paths, not only to node paths.

and which is more appropriated to get the node ID in a block preprocess function.

If all you want to get is the node ID of the node currently shown, then you just need to use the following code:

if (arg(0) == 'node') {
  $nid = arg(1);
}

If you need to get the node object, then you can use the following code:

if ($node = menu_get_object()) {
  // Use the node object.
}

Of those snippets, the second is better, as it returns the correct node object for internal paths such as node/1/revisions/3/view.

Keep in mind that some preprocess functions, such as the ones for the page, or the node, have already a $variables['node']. This means that in those cases, you just need to use the following snippet in your preprocess function, without worrying about which function to use.

if (isset($variables['node'])) {
  // Check the node ID or other properties.
}

For other preprocess functions, check the documentation to see if there is any variable containing the node object. For example, that is the case for template_preprocess_comment() which makes $variables['node'] available to the preprocess functions for comments.

How it's done in Drupal Core's template_preprocess_page:

Line 2267 of theme.inc and onwards:

if ($node = menu_get_object()) {
    $variables['node'] = $node;
}

template_preprocess_page uses menu_get_object. After that it's just a matter of checking if the node is defined and getting $node->nid.

One thing you have to be careful of is path alias.

drupal_get_normal_path($path)

to get the current path use the system variable $_SERVER["REQUEST_URI"]

(If you are on the node page you can call $node->uri['path'], this will return a non-alias path. Then again if you are on the node page, you can just use $node->nid.)

this will return node/XXXX

so try something like

  $node_path = explode('/', drupal_get_normal_path($path));
  $nid = $node_path[1];