src/AESKW.php
private static function getInitialValue(string &$key, bool $padding_enabled): string
    {
        if (false === $padding_enabled) {
            return \hex2bin('A6A6A6A6A6A6A6A6');
        }

        $MLI = \mb_strlen($key, '8bit');
        $iv = \hex2bin('A65959A6').self::toXBits(32, $MLI);

        $n = \intval(\ceil($MLI / 8));
        $key = \str_pad($key, 8 * $n, "\0", STR_PAD_RIGHT);

        return $iv;
    }
private static function getInitialValue(string &$key, bool $padding_enabled): string
    {
        if (false === $padding_enabled) {
            return \hex2bin('A6A6A6A6A6A6A6A6');
        }

        $MLI = \mb_strlen($key, '8bit');
        $iv = \hex2bin('A65959A6').self::toXBits(32, $MLI);

        $n = \intval(\ceil($MLI / 8));
        $key = \str_pad($key, 8 * $n, "\0", STR_PAD_RIGHT);

        return $iv;
    }
The initial value used to wrap the key and check the integrity when unwrapped.
The RFC3394 set this value to 0xA6A6A6A6A6A6A6A6
The RFC5649 set this value to 0xA65959A6XXXXXXXX (The part with XXXXXXXX is the MLI, depends on the padding).

@param string $key             The key
@param bool   $padding_enabled Enable padding (RFC5649)

@see https://tools.ietf.org/html/rfc3394#section-2.2.3.1