Debian Bullseye Stable 11 uses PHP 7.4 link . It is still the Stable Debian release for all production Debian servers until middle of 2023. And then you can use Debian Bullseye OldStable one year and then Debian Bullseye LTS another year.
PHP 8.1 link is only in Debian Bookworm Testing/Sid 12. Of course you can always include PHP 8.1 as a third-party source.