Here you'll find more infos:
http://www.sgm24.com/category/mobile-redirect/
We have evaluated PHP vs. .htaccess vs. JavaScript with a lot of our affiliates.
A PHP redirect is officially recommended by Google.
Here Google's official statement:
https://youtube.com/watch?v=hK6wf7CBYS8
Our webmasters haven't seen any punishments by redirecting via .htaccess
Within 2 days a lot of webmasters lost their pretty decent Google ranking by redirecting via JavaScript.
Also not all devices (especially older ones) do support JavaScript.