Rule source on GitHub

DowngradeVoidCastRector

Replace void casts with proper handling of return values

 #[\NoDiscard]
 function getPhpVersion(): string
 {
     return 'PHP 8.5';
 }

-(void) getPhpVersion();
+$_void = getPhpVersion();

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp85\Rector\Expression\DowngradeVoidCastRector;

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