Comment by chuckadams

One form of type checking does happen at compile time (which is really load time in PHP, but close enough), namely when a class extends another class or implements an interface: the types of every method and property are checked to ensure that they are substitutable per the standard variance rules (return types are covariant, parameters are contravariant, props are invariant). Everything else is checked at runtime though, and statically analyzing any of those is left to external tools like phpstan, psalm, or mago.