Reference: Return Type Conflict vs. #[\ReturnTypeWillChange] Attribute

Context:
In PHP 8.1, specifying return types for methods becomes more prevalent, leading to conflicts with existing implementations.

Problem:
When a method's return type changes from a compatible type to an incompatible type or is unspecified, the following deprecation notice appears:

Deprecated: Return type of [Method Name] should either be compatible with ..., or the #[\ReturnTypeWillChange] attribute should be used...

Implications of Return Type Covariance:
PHP 7.0 introduced return types, ensuring consistent behavior for calling code based on the specified contract. Extension or implementation classes must maintain the same or more specific return types.

Background of Deprecation:
With the addition of Union Types in PHP 8.0, internal functions and methods gained return type specifications. However, enforcing these types would break backward compatibility. Instead, a deprecation notice was introduced for tentative changes.

Purpose of #[\ReturnTypeWillChange] Attribute:
This attribute indicates a planned change in return type. PHP 8.1 ignores the deprecation notice for methods marked with this attribute, allowing support for older PHP versions.

What To Do:

1. Identify the method mentioned in the deprecation notice and the expected return type.
2. If possible, change the return type of the method to match the expected type.
3. If necessary, suppress the deprecation notice temporarily by adding the #[\ReturnTypeWillChange] attribute.

Note: Enforced return types are likely in PHP 9.0, so plan to address #[\ReturnTypeWillChange] attributes accordingly.