Return Type Will Change

This is a native PHP attribute. It tells the engine that method overwriting or implementing a native method will use a different return type.

PHP checks the compatibility of implementing methods versus a custom interface. Until PHP 8.1, it did not report such violation when the interface is a PHP native one: this was for backward compatibility.

To avoid using that attribute, one must made the return type compatible with PHP signatures.

This attributes only works with PHP native methods: it is ignored when used with other methods.

<?php

class Foo implements ArrayAccess {
    #[\ReturnTypeWillChange]
    public function offsetGet(mixed $offset) {}
    // ...
}

?>

Documentation

Related : PHP Native Attribute

Added in PHP 8.1+