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             return 'Todo not found';
         }
   
-        if (!array_key_exists($this->todos[$index]['unique_key'], $this->selectedTodos)) {
+        if (!array_key_exists((string) $this->todos[$index]['unique_key'], $this->selectedTodos)) {
             $this->selectedTodos[$this->todos[$index]['unique_key']] = $this->todos[$index]['label'];
             return 'Todo selected !';
         }

PHP snippet to change

<?php

final class DemoFile
{
    /**
     * @var array<int, array{unique_key: int|string, label: string}>
     */
    private array $todos = [
        0 => [
            'unique_key' => 1,
            'label' => 'test 1',
        ],
        1 => [
            'unique_key' => 'fallback_todo',
            'label' => 'fallback todo 1',
        ],
    ];
    
    /**
     * @var array<int|string, string>
     */
    public array $selectedTodos = [];

    public function run(int $index): string
    {
  		if (!array_key_exists($index, $this->todos)) {
            return 'Todo not found';
        }
  
        if (!array_key_exists($this->todos[$index]['unique_key'], $this->selectedTodos)) {
            $this->selectedTodos[$this->todos[$index]['unique_key']] = $this->todos[$index]['label'];
            return 'Todo selected !';
        }

        return 'Todo already selected !';
    }
}

Applied Rules:

ArrayKeyExistsNullToEmptyStringRector

Not a change you expect?

Create an issue on GitHub
...or to speed up fix – add a test fixture

Config rector.php

<?php

declare(strict_types=1);

use Rector\Config\RectorConfig;
use Rector\Php85\Rector\FuncCall\ArrayKeyExistsNullToEmptyStringRector;
use Rector\ValueObject\PhpVersion;

return static function (RectorConfig $rectorConfig): void {
    $rectorConfig->rule(ArrayKeyExistsNullToEmptyStringRector::class);

    $rectorConfig->phpVersion(PhpVersion::PHP_85);
};

Rector version: 40d87a - released at 2025-12-24 10:28