Use of an undeclared variable

int_meta_show_tags() calls itself with the value of a variable which is not before declared, and to which is not given any values; the passed value is not even needed, as the function declares just one argument, in its formal parameters list.

function int_meta_show_tags($tag) {
  static $types = array();
  if (is_array($tag)) {
    // Cycle through each tag individually
    foreach ($tag as $curr) {
      int_meta_show_tags($curr, $empty);
    }
    // ...
}