Rule source on GitHub

AddReturnTypeWillChangeAttributeRector

Add #[\ReturnTypeWillChange] attribute to methods implementing PHP 8.1 interface methods with provisional return types, to suppress deprecation notices when running on PHP 8.1+ without the required return types

 class SomeClass implements \ArrayAccess
 {
+    #[\ReturnTypeWillChange]
     public function offsetGet($offset)
     {
     }
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp81\Rector\ClassMethod\AddReturnTypeWillChangeAttributeRector;

return RectorConfig::configure()
    ->withRules([
        AddReturnTypeWillChangeAttributeRector::class,
    ]);