we should use type declarations in PHP functions

[cgolubi1]

With more recent PHP versions, we have access to type declarations - https://www.php.net/manual/en/language.types.declarations.php - enforced type settings on function parameters which will cause failures if they mismatch.

This is great, because situations in which we're wrong about the type of the parameter we're passing to a function, are a source of bugs for us.

Once we upgrade the PHP version to 8.3 in #3075, we should define type declarations for all our functions, and figure out how to make CircleCI tests enforce that future functions will also define them.