-reset($items);
-$firstKey = key($items);
+$firstKey = array_key_first($items);

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\Php73\Rector\FuncCall\ArrayKeyFirstLastRector;

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

SETS:  PHP 7.3 Polyfills