Rule source on GitHub

DowngradeHashAlgorithmXxHashRector

Downgrade hash algorithm xxh32, xxh64, xxh3 or xxh128 by default to md5. You can configure the algorithm to downgrade.

 class SomeClass
 {
     public function run()
     {
-        return hash('xxh128', 'some-data-to-hash');
+        return hash('md5', 'some-data-to-hash');
     }
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp81\Rector\FuncCall\DowngradeHashAlgorithmXxHashRector;

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