Rule source on GitHub

DowngradeDomNodeChildNodesForeachRector

Add null coalesce to a foreach over DOMNode::$childNodes, as it can be null before PHP 8.0

 function run(\DOMNode $node)
 {
-    foreach ($node->childNodes as $childNode) {
+    foreach ($node->childNodes ?? [] as $childNode) {
         echo $childNode->nodeValue;
     }
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp80\Rector\Foreach_\DowngradeDomNodeChildNodesForeachRector;

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