Using PHP8 Static Return Type to Precisely Declare Static Method Return Types

PHP8 as an object‑oriented scripting language provides many new features and improvements. One important change is the enhanced type declarations, especially for static method return types. In this article we explore how to use PHP8's new feature — Static Return Type — to better declare static method return types, with concrete code examples.

In previous PHP versions we could use return type declarations to specify the return value type of functions or methods, such as int, string, array, etc. In PHP8 we can use Static Return Type to declare the return type of a static method. This means we can tell the caller that the function returns an instance of a specific class, not just the class or its subclasses.

Next we will use a simple example to demonstrate how to use Static Return Type to declare a static method's return type. Suppose we have a class named User with a static method getById that returns a User instance based on a user ID. The example code is:

class User {
    public static function getById(int $id) : static {
        // Based on ID, query user information
        // ...

        // Create User instance and return
        return new static();
    }
}

$user = User::getById(1);

In the above example we use Static Return Type to declare that getById returns a User instance. By using the static keyword we ensure the returned instance is of the actual class that invoked the method.

This static return type declaration has an important benefit: when overriding a parent class's static method in a subclass, the subclass's return type automatically adapts. For example, if we create a subclass Admin and override getById, the static return type will automatically become an Admin instance. Example code:

class Admin extends User {
    public static function getById(int $id) : static {
        // Based on ID, query admin information
        // ...

        // Create Admin instance and return
        return new static();
    }
}

$user = Admin::getById(1);

In this example we created an Admin class and overrode getById. Although we did not explicitly declare the return type as Admin, because we used Static Return Type, the returned instance automatically adapts to an Admin instance.

Another important feature is flexibility when combined with inheritance and polymorphism. We can use static return type to return subclass instances from a parent class's static method, which is useful in certain scenarios. Example code:

class Factory {
    public static function createUser() : User {
        // Create User instance and return
        return new User();
    }
}

class AdminFactory extends Factory {
    public static function createUser() : Admin {
        // Create Admin instance and return
        return new Admin();
    }
}

$user = Factory::createUser(); // returns User instance
$admin = AdminFactory::createUser(); // returns Admin instance

In the above example we created a Factory class with a static method createUser that returns a User instance. Then we created an AdminFactory subclass that overrides createUser to return an Admin instance. By using Static Return Type we can return subclass instances from a parent class's static method, achieving more flexible object creation.

Summary

PHP8's Static Return Type feature provides a better way to declare static method return types. It allows precise specification of the returned instance type and offers flexibility when combined with inheritance and polymorphism. Using Static Return Type improves code readability and maintainability.