Rule source on GitHub

DowngradeSetAccessibleReflectionPropertyRector

Add setAccessible() on ReflectionProperty to allow reading private properties in PHP 8.0-

 class SomeClass
 {
     public function run($object)
     {
         $reflectionProperty = new ReflectionProperty($object, 'bar');
+        if (PHP_VERSION_ID < 80100) {
+            $reflectionProperty->setAccessible(true);
+        }

         return $reflectionProperty->getValue($object);
     }
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp81\Rector\StmtsAwareInterface\DowngradeSetAccessibleReflectionPropertyRector;

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