Rule source on GitHub

DowngradeOctalNumberRector

Downgrades octal numbers to decimal ones

 class SomeClass
 {
     public function run()
     {
-        return 0o777;
+        return 0777;
     }
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp81\Rector\LNumber\DowngradeOctalNumberRector;

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