request_uri

The standard Drupal path is often ugly. Enter the path module, which lets us assign a friendlier alias path. Both the alias and the "real" path, typically called the Drupal path or the internal path, can be used to access the page. Internally Drupal always uses the Drupal path, regardless of which path was used for the original access. For example, if you assign the alias /about to /node/3, it's still /node/3 to Drupal.

The arg function gives us the Drupal path of the current page as component bits. For /node/3, the arg function returns arg(0) = node and arg(1) = 3. Even if the page is accessed via /about, the arg function returns arg(0) = node and arg(1) = 3. To get the Drupal path as a string, you can use $_GET['q'], which would return the string /node/3.