class HelloWorld
{
    private const TYPES = [
        'foo' => DateTime::class,
        'bar' => DateTimeImmutable::class,
    ];

    /**
     * @template T of key-of<self::TYPES>
     * @param T $type
     * @return self::TYPES[T]
     */
    public static function get(string $type) : ?object
    {
        $class = self::TYPES[$type] ?? null;
        if ($class === null) {
            return null;
        }

        return new $class('now');
    }
}

@return (self::TYPES[T] is defined ? self::TYPES[T] : null)