What is the best way to detect IE browsers 8 and below?

Question

I'm using some HTML5 functionality that only works on IE9 and I need to detect whether visitors are running on IE8 or below (mainly just IE8 or 7 of course). I've tried some plugins but the one that was working didn't seem to support IE9 (just up to 8). It's called PHP Browser Detection (http://wordpress.org/extend/plugins/php-browser-detection/)