CI_Encryption::substr PHP Méthode

    protected static function substr($str, $start, $length = NULL)
    {
        if (self::$func_override) {
            // mb_substr($str, $start, null, '8bit') returns an empty
            // string on PHP 5.3
            isset($length) or $length = $start >= 0 ? self::strlen($str) - $start : -$start;
            return mb_substr($str, $start, $length, '8bit');
        }
        return isset($length) ? substr($str, $start, $length) : substr($str, $start);
    }