Rule source on GitHub

DowngradeArrayKeyFirstLastRector

Downgrade array_key_first() and array_key_last() functions

 class SomeClass
 {
     public function run($items)
     {
-        $firstItemKey = array_key_first($items);
+        reset($items);
+        $firstItemKey = key($items);
     }
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp73\Rector\FuncCall\DowngradeArrayKeyFirstLastRector;

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