Registry.php

<?php

namespace DesignPatterns\Structural\Registry;

abstract class Registry
{
    const LOGGER = 'logger';

    /**
     * this introduces global state in your application which can not be mocked up for testing
     * and is therefor considered an anti-pattern! Use dependency injection instead!
     *
     * @var array
     */
    private static $storedValues = [];

    /**
     * @var array
     */
    private static $allowedKeys = [
        self::LOGGER,
    ];

    /**
     * @param string $key
     * @param mixed  $value
     *
     * @return void
     */
    public static function set(string $key, $value)
    {
        if (!in_array($key, self::$allowedKeys)) {
            throw new \InvalidArgumentException('Invalid key given');
        }

        self::$storedValues[$key] = $value;
    }

    /**
     * @param string $key
     *
     * @return mixed
     */
    public static function get(string $key)
    {
        if (!in_array($key, self::$allowedKeys) || !isset(self::$storedValues[$key])) {
            throw new \InvalidArgumentException('Invalid key given');
        }

        return self::$storedValues[$key];
    }
}