suggestion on how to reduce execution frequency

[chnagy]

Just wanted to leave my suggestion to reduce the frequency of the api calls.

Many modern router offer a dyndns feature.
I published a dummy GET endpoint in my local network which executes your update.php and entered the host as dyndns server in my router so it triggers the update script only when I receive a new IP from my ISP.

[DarkSpir]

Also a good idea would be to compare the current IP address (as returned by getCurrentPublicIPv[4|6] functions) to the result of a DNS [A|AAAA] record resultion (see dns_get_record function in PHP). If they match, you don't even have to log into the Netcup API because there's nothing for you to do anyway.