Rule source on GitHub

DowngradeNumericLiteralSeparatorRector

Remove "_" as thousands separator in numbers

 class SomeClass
 {
     public function run()
     {
-        $int = 1_000;
-        $float = 1_000_500.001;
+        $int = 1000;
+        $float = 1000500.001;
     }
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp74\Rector\LNumber\DowngradeNumericLiteralSeparatorRector;

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