Drupal - Getting node id from view to customise link in block

Here is a more-robust way of getting the node ID:

<?php
    // Check that the current URL is for a specific node:
    if(arg(0) == 'node' && is_numeric(arg(1))) {
        return arg(1); // Return the NID
    }
    else { // Whatever it is we're looking at, it's not a node
      return NULL; // Return an invalid NID
    }
?>

This method works even if you have a custom path for your node with the path and/or pathauto modules.

Just for reference, if you don't turn on the path module, the default URLs that Drupal generates are called "system paths" in the documentation. If you do turn on the path module, you are able to set custom paths which are called "aliases" in the documentation.

Since I always have the path module turned on, one thing that confused me at first was whether it was ever possible for the arg function to return part of an alias rather than part of system path.

As it turns out, the arg function will always return a system path because the arg function is based on $_GET['q']... After a bit of research it seems that $_GET['q'] will always return a system path.

If you want to get the path from the actual page request, you need to use $_REQUEST['q']. If the path module is enabled, $_REQUEST['q'] may return either an alias or a system path.

For a solution, especially one that involves a view argument in the midst of a path like department/%/list, see the blog post Node ID as View Argument from SEO-friendly URL Path.

In the end this snippet did the job - it just stripped the clean URL and reported back the very last argument.

<?php
    $refer= $_SERVER ['REQUEST_URI'];
    $nid = explode("/", $refer);
    $nid = $nid[3];
?>

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Given the comment reply, the above was probably reduced to this, using the Drupal arg() function to get a part of the request path:

<?php
    $nid = arg(3);
?>