Problems with libicu65 - Installing php5.6 on bookworm

I have one ISPconfig slave server based on bookworm (Deb 12).

When trying to install php 5.6 following the advices on https://www.howtoforge.com/ispconfig-php-debian/) (sorry, need that for upgrading old owncloud-instances to the 2026 level) like I did SEVERAL time with other combinations of Debian OS/PHP versions (no problems with bullseye at all!) I ran into this problem:
E: Paket libicu65 kann nicht gefunden werden.
I don't find any hints how to deal with this. "Downgrading" to bullseye as the basic OS on that server is no option, right? I really thought about that, too...

 

Maybe that package does not exist anymore on Sury package repository.
Try leaving that package name out from the install command. Maybe system works without it nowadays.
If that does not help, verify you have entered the Sury package repository correctly to sources.list and run apt update before the install.