<?php

function sayHello(string $name): never
{
    echo "Hello, $name";
    exit(); // if we comment this line, php throws fatal error
}

sayHello("John"); // result: "Hello, John"

I think the description should be corrected from return-only to non-return function since the context is now misleading

Never cannot be used in a union type because, being the bottom type, it is already automatically a subtype of every other type. "A|never" is equivalent to "A".

When one type is "obviously" a subtype of another (i.e., it doesn't require loading the class definitions of all the types involved), the former is redundant in union types, and PHP flags the union of both as an error.

Similarly for intersection types, where "A&never" means the same thing as "never". It "obviously" doesn't make sense to mention A there, so PHP won't allow doing so.