node reference in "Specify the fields value with PHP code"

Hello; I created a Triggered rule

Created content's field 'field_xxx' has value

I must set a value : OK, no problem

but I want to use : Advanced: Specify the fields value with PHP code

For example, this works:

return array(
0 => array('nid' => 24),
);
but I must use a node referrence

$noderefnode = node_load($field_yyy[0][nid]);
$noderefnode->field_zzz[0]['nid']

How to integrate the code above with return array.... (the php code of the value...)

Any idea? Thanks :)