Rule source on GitHub

DowngradeNullCoalescingOperatorRector

Remove null coalescing operator ??=

 $array = [];
-$array['user_id'] ??= 'value';
+$array['user_id'] = $array['user_id'] ?? 'value';

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp74\Rector\Coalesce\DowngradeNullCoalescingOperatorRector;

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