New never Return Type in PHP 8.1

PHP allows defining type declarations for parameters, return values, and properties. Since PHP 8.1, we can use never return type. It indicates that a function or class method throws an exception or terminates execution of the script by calling die or exit.

<?php

function redirectToPage(string $url): never
{
    header('Location: '.$url);
    exit;
}

The never return type is different from void. The void return type indicates that a function or class method doesn't return a value. The never return type guarantees that a function or class method throws an exception or terminates the script. If it doesn't, PHP produces a fatal error:

<?php

function saySomething(string $message): never
{
    echo $message;
}

saySomething('Hello');

Fatal error: Uncaught TypeError: saySomething(): never-returning function must not implicitly return in ...

There are some important notes:

• The never type is only allowed to use as return type. It cannot be used to define type for parameters and properties.
• A function or class method which declared with never return type cannot call return statement, and not even return; that doesn't specify return value.
• The never type cannot be used as part of the union types. For example, string|never is not allowed and cannot be used to define return type.