To be honest, considering your other post that I just responded to, I don't really feel like taking the time out to answer this post. But I'll put my feelings aside as even if you don't value my time and energy, hopefully someone else reading this might.

TurnKey v17.x is based on Debian 11/Bullseye. Debian ships with PHP 7.4 - with backported security support until 2027. That's why we can support automated security updates.

Ondřej Surý (a Debian and Ubuntu developer) provides a third party apt repository which supports installing alternate versions of PHP on Debian. He is a trusted developer, but because his repo provides PHP upstream code, it doesn't include the safe, carefully backported packages that the Debian security team provides. Hence why automated security updates aren't enable for the 3rd party PHP repo.

Core itself does not include PHP, but PHP7.4 is installable direct from the Debian repos. If you want newer PHP (up to PHP8.2), then you can add it yourself (as discussed elsewhere). Whilst most people seem to be able to get that working, others do seem to be having problems with it. As I've mentioned elsewhere, I'm more than happy to assist anyone having problems with setting up alternate PHP versions, but I can't do it telepathically, so I need some info from you! :)

If you wish to develop a new appliance, then you could leverage what we've done on TKLDev, to include an alternate PHP version at build time. You could use that to create a LAMP server with PHP8.1. And discussing this, it occurs to me that, that might actually be an appliance worth creating (LAMP with latest PHP)! I'll think about that a bit more. In the meantime, there are a couple of appliances that include PHP8.1 out of the box that you could use as reference if you wanted to? E.g. the CakePHP appliance.

Actually, now I'm thinking about it, you could just use the CakePHP appliance as a base (just remove the CakePHP app) for installing what you want? That should be pretty easy and you have PHP 8.1 pre-installed and ready to go.

I'm more than happy to provide further, more explicit guidance if you wish. Please just ask.