I did not understand you question (perhaps, we can discuss offline).

But, looking at it again, the usual (more elegant and easier to read) construction (I think) would be:

$final_result = False; // or some default behaviour
if($old_bookmark['exists']) { // Also, if the variable is boolean, there is no need to check if it's equals to True
  storeTags($old_bookmark['bId'], $tags_str);
} else {
  $result = add_bookmark($username, $title_str, 0, $url_str, '', $tags_str, True, $date_str);
  $final_result = $result['success'];
}
return $final_result;