Do a 307 temporary redirect on the server side. You can do this with PHP or .htaccess, it doesn't matter which. But definitely make sure you send the 307 header as that is what tells the search engines the redirect is temporary and to try again later. Without it they will consider the pages removed which is exactly what you don't want.
header('Location: /forum', true, 307); exit;
header()). This will ensure that the original homepage maintains it's pagerank while it's under construction.
But really, can't you tinker with it on a test environment and just upload it to the live website once you're done? That's a lot more SEO-efficient.