What do the question marks do before the types of variables in the parameters?

Question

Could you please tell me how is this called? ?string and string

Usage example:

public function (?string $parameter1, strin         


        

Answer 1

This is roughly equivalent to

 public function (string $parameter1 = null, string $parameter2) {}

Except that the argument is still required, and an error will be issued if the argument is omitted.

Specifically in this context, the second argument is required and using =null would make the first optional, which doesn't really work. Sure it works but what I mean that it does not actually make it optional, which is the main purpose of default values.

So using

public function (?string $parameter1, string $parameter2) {}

Syntactically makes a bit more sense in this instance.