<?php
function sayHello(string $name): never
{
echo "Hello, $name";
exit(); // if we comment this line, php throws fatal error
}
sayHello("John"); // result: "Hello, John"
never is a return-only type indicating the function does not terminate. This means that it either calls exit(), throws an exception, or is an infinite loop. Therefore, it cannot be part of a union type declaration. Available as of PHP 8.1.0.
never is, in type theory parlance, the bottom type. Meaning it is the subtype of every other type and can replace any other return type during inheritance.
<?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 misleadingNever 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.