code: public static function isUtf8($str, &$special = false)
      {
          self::setStr($str);
          $special = false;
          while (self::$curI < self::$stats['strLen']) {
              $isUtf8 = self::isOffsetUtf8($isSpecial); // special is only checking control chars
              if (!$isUtf8) {
                  return false;
              }
              if ($isSpecial) {
                  $special = true;
              }
          }
          $special = $special || self::hasSpecial($str);
          return true;
      }
docs: Determine if string is UTF-8 encoded
  
  In addition, if valid UTF-8, will also report whether string contains
  control, or other speical characters that could otherwise go unnoticed
  
  @param string  $str     string to check
  @param boolean $special does valid utf-8 string control or "exotic" whitespace type character
  
  @return boolean