public static function substringAfter($str, $separator)
    {
        if (true === self::isEmpty($str)) {
            return $str;
        }

        if (null === $separator) {
            return self::EMPTY_STR;
        }

        $pos = self::indexOf($str, $separator);

        if (self::INDEX_NOT_FOUND === $pos) {
            return self::EMPTY_STR;
        }

        return self::substring($str, $pos + self::length($separator));
    }