Rule source on GitHub

DowngradeArbitraryExpressionsSupportRector

Replace arbitrary expressions used with new or instanceof

 function getObjectClassName() {
     return stdClass::class;
 }

-$object = new (getObjectClassName());
+$className = getObjectClassName();
+$object = new $className();

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp80\Rector\New_\DowngradeArbitraryExpressionsSupportRector;

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