Tök egyszerű. Nem?

$isEmpty = !(bool)
  trim(
    trim(
      strip_tags(
        html_entity_decode(
          mb_convert_encoding(
            $str,
            'UTF-8',
            mb_detect_encoding($str)
          ),
          ENT_COMPAT,
          'UTF-8'
        )
      )
    ),
    chr(0xC2).chr(0xA0)
  );