ParagonIE_Sodium_Core_Util::substr

    public static function substr($str, $start = 0, $length = null)
    {
        /* Type checks: */
        if (!is_string($str)) {
            throw new TypeError('String expected');
        }

        if ($length === 0) {
            return '';
        }

        if (self::isMbStringOverride()) {
            if (PHP_VERSION_ID < 50400 && $length === null) {
                $length = self::strlen($str);
            }
            $sub = (string) mb_substr($str, $start, $length, '8bit');
        } elseif ($length === null) {
            $sub = (string) substr($str, $start);
        } else {
            $sub = (string) substr($str, $start, $length);
        }
        if ($sub !== '') {
            return $sub;
        }
        return '';
    }