SMTP::hmac( string $data, string $key ): string

702    protected function hmac($data, $key)703    {704        if (function_exists('hash_hmac')) {705            return hash_hmac('md5', $data, $key);706        }707 708        //The following borrowed from709        //https://www.php.net/manual/en/function.mhash.php#27225710 711        //RFC 2104 HMAC implementation for php.712        //Creates an md5 HMAC.713        //Eliminates the need to install mhash to compute a HMAC714        //by Lance Rushing715 716        $bytelen = 64; //byte length for md5717        if (strlen($key) > $bytelen) {718            $key = pack('H*', md5($key));719        }720        $key = str_pad($key, $bytelen, chr(0x00));721        $ipad = str_pad('', $bytelen, chr(0x36));722        $opad = str_pad('', $bytelen, chr(0x5c));723        $k_ipad = $key ^ $ipad;724        $k_opad = $key ^ $opad;725 726        return md5($k_opad . pack('H*', md5($k_ipad . $data)));727    }