Make my self hosted webpage accessible to mobile phones only

So I've already looked at these questions asked on stackoverflow with no joy :(

1. Can you make a webpage accessible only by mobile application?
2. Limiting site access to Mobile Phones
3. Only show webpage to mobile browsers

I'm creating a site that will only work if you have a smartphone, as it interacts with its features. I don't want to waste desktop searchers time therefore I would like to display a page saying "Only view-able via smartphones" if it detects a non smartphone visitor. Its important that desktops viewers cannot go any further into the site, so the detection has to happen asap

Any help would be great.

Thanks.