PHP – If a function parameter has a default value of null, should its type be declared as nullable?

Is it a best practice to mark as nullable (?string) the type of parameters that have a default value of null, given that the parameter is always passed as a string, for example, or some other non null value? The default value of null is only specified to make the parameter optional.

function example(?string $param = null) {
    // Function logic here
}