Rule source on GitHub

DowngradeArrayFilterNullableCallbackRector

Unset nullable callback on array_filter

 class SomeClass
 {
     public function run($callback = null)
     {
         $data = [[]];
-        var_dump(array_filter($data, null));
+        var_dump(array_filter($data));
     }
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp80\Rector\FuncCall\DowngradeArrayFilterNullableCallbackRector;

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