voku / portable-utf8

      $haystack = self::clean($haystack);

    }

    if (self::$support['mbstring'] === true) {

      // INFO: this is only a fallback for old versions

      if ($encoding === true || $encoding === false) {

        $encoding = 'UTF-8';

      } else {

        $encoding = self::normalizeEncoding($encoding);

      }

      return \mb_strpos($haystack, $needle, $offset, $encoding);

    }

    if (self::$support['iconv'] === true) {

      // ignore invalid negative offset to keep compatility

      $length = (int)$length;

    }

    if (self::$support['mbstring'] === true) {

      // INFO: this is only a fallback for old versions

      if ($encoding === true || $encoding === false) {

        $encoding = 'UTF-8';

      } else {

        $encoding = self::normalizeEncoding($encoding);

      }

      return \mb_substr($str, $start, $length, $encoding);

    }

    if (self::$support['iconv'] === true) {

      return (string)\grapheme_substr($str, $start, $length);