firefly-iii/app/Support/Singleton/PreferencesSingleton.php

<?php

namespace FireflyIII\Support\Singleton;

class PreferencesSingleton
{
    private static ?PreferencesSingleton $instance = null;

    private array $preferences = [];

    private function __construct()
    {
        // Private constructor to prevent direct instantiation.
    }

    public static function getInstance(): PreferencesSingleton
    {
        if (self::$instance === null) {
            self::$instance = new PreferencesSingleton();
        }

        return self::$instance;
    }

    public function setPreference(string $key, mixed $value): void
    {
        $this->preferences[$key] = $value;
    }

    public function getPreference(string $key): mixed
    {
        return $this->preferences[$key] ?? null;
    }

}