PHP contextual filter for taxonomy ID

I´m using the following code to retrieve the taxonomy id from the URL and use it as a contextual filter.
When I use it on a computed field as it is shown below it works OK.

$nid = arg(1);
$path_alias = drupal_get_path_alias('node/' . $nid);
$partes = explode("/", $path_alias);
$dato = $partes[2];
$entity_field[0]['value'] = $dato;

When I use the same code on a views contextual filter it doesn´t work:

$nid = arg(1);
$path_alias = drupal_get_path_alias('node/' . $nid);
$partes = explode("/", $path_alias);
$dato = $partes[2];
return $dato;

If I replace line 4 to force a value into $dato it works OK too, of course for only one taxonomy term.

$nid = arg(1);
$path_alias = drupal_get_path_alias('node/' . $nid);
$partes = explode("/", $path_alias);
$dato = 43;
return $dato;

I would like to receive any help on what I need to correct on the second example to make it work as a contextual filter.
Thanks!