Rule source on GitHub

DowngradeNumberFormatNoFourthArgRector

Downgrade number_format arg to fill 4th arg when only 3rd arg filled

 class SomeClass
 {
     public function run()
     {
-        return number_format(1000, 2, ',');
+        return number_format(1000, 2, ',', ',');
     }
 }

Configure your rector.php:

<?php

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

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