Rule source on GitHub

DowngradePhp81ResourceReturnToObjectRector

change instanceof Object to is_resource

 class SomeClass
 {
     public function run($obj)
     {
-        $obj instanceof \finfo;
+        is_resource($obj) || $obj instanceof \finfo;
     }
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp81\Rector\Instanceof_\DowngradePhp81ResourceReturnToObjectRector;

return RectorConfig::configure()
    ->withRules([
        DowngradePhp81ResourceReturnToObjectRector::class,
    ]);