How to resolve the "undefined variable" notce when I put a flag on a Views page?
The error arises because there is no $node variable defined in the scope.
Try looking inside the $results variable; something like this should work:

&lt;?php print flag_create_link('subscribe', $results[0]-&gt;nid); ?&gt;


This will print a subscribe link for the first row of the view. 
Also, you have to ensure that the nid is present in your result object. It should be already there, but if not, add the "Content: Nid" field and hide it by checking the "Exclude from display" option.