src/S.php
public static function lower($string)
    {
        if (function_exists('mb_strtolower')) {
            return mb_strtolower($string, static::getEncoding());
        }

        return static::replaceByMap($string, static::$cyrillicAlphabet[0], static::$cyrillicAlphabet[1]);
    }
public static function lower($string)
    {
        if (function_exists('mb_strtolower')) {
            return mb_strtolower($string, static::getEncoding());
        }

        return static::replaceByMap($string, static::$cyrillicAlphabet[0], static::$cyrillicAlphabet[1]);
    }
Lower case.
@param $string
@return bool|string