Solus does not use PHP 8.x as we provide some development tooling (namely Arcanist, for the Phabricator-based development tracker) which only support PHP 7. You will need to compile it from source, you may be able to use our PHP package as a basis for this work.
My suggestion would be to also rename the php package you have locally (via the package.yml) when you are modifying it, to
php8 so there is not a naming conflict with ours.
In the event there is no file conflicts between the PHP 7.x and PHP 8.x packages (verifiable using the pspec) I am open to having both in the repository, assuming there is a valid case for needing it locally.