NullCoalescingOperatorRector

Use null coalescing operator ??=

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

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\Php74\Rector\Assign\NullCoalescingOperatorRector;

return RectorConfig::configure()
    ->withRules([
        NullCoalescingOperatorRector::class,
    ]);
SETS:  PHP 7.4