USER_AGENT will tell you what the browser claims it is - but most browsers can be made to lie. get_browser does the same thing, it looks up the capabilities of the browser in a file on the server, using the data the browser claims. jQuery's "support" is better, since it tells you whether the browser supports the function you need to know about. Just send the information from the client to the PHP page (use AJAX). (Regardless of what the browser claims to be, support reports what the browser's engine is capable of.)
What's the information you're trying to get? PHP is not able to pass on all properties of the browser.