How Should I Handle Return Type Deprecation Notices in PHP?

Reference: Return type of ... should either be compatible with ..., or the #[ReturnTypeWillChange] attribute should be used

Background: Return types and covariance

Since PHP 7.0, return types for functions and methods have been available. This forms a contract that other code can rely on.

For example:

<code class="php">class Base {
    public function getList(): Iterator {
       // ...
    }
}

// Calling code:
$foo = new Base();
var_dump($foo instanceOf Base); // true
var_dump($foo->getList() instanceOf Iterator); // true</code>

When extending a class, a more specific return type can be defined (covariance). However, specifying a different return type or no return type is not allowed.

Backwards compatibility and deprecation

Adding return types to existing classes or interfaces requires changes in extending or implementing classes. With the introduction of Union Types in PHP 8.0, return types were documented but not enforced as errors. Instead, a deprecation notice was added.

The #[ReturnTypeWillChange] attribute

Conflicting code that needs to support multiple PHP versions and pre-8.0 return types led to the development of the #[ReturnTypeWillChange] attribute. When added to a method, it suppresses the deprecation notice in PHP 8.1. This gives time to rectify the issue before PHP 9.0, where internal return types may be enforced.

Resolution

1. Identify the method and correct return type from the depreciation notice.

2. Decide your course of action:

   • Wait for a library or extension update (no action required).
   • Check if the class already returns the correct type.
   • Determine if changing the return type is safe for your application and extending classes.

3. If it's safe to change the return type:

   • Add the return type.

4. If you need to support older PHP versions or un-updated code:

   • Add the #[ReturnTypeWillChange] attribute temporarily.

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