In the documentation for photos_search() function, you'll see that I tell you to use a space delimited string if you're searching for tags. Unless I am sorely mistaken, that's the way it used to work. What I know for sure is that Flickr now wants a comma-delimited string for tags. Which means that it should read:
$photos = $f->photos_search(array("tags"=>"brown,cow", "tag_mode"=>"any"));