web phone app

Check this for mobile browser IDs so you can do the other way:

http://www.zytrax.com/tech/web/mobile_ids.html

Here is a PHP example:

http://php.net/manual/en/function.get-browser.php

This information is "external data" and may be subject to tampering.  That said, look at this variable:
$_SERVER['HTTP_USER_AGENT']

You can find it if you run phpinfo();

The notes on the get-browser page are useful, and see especially this note (looks like get-browser has some problems):
http://www.php.net/manual/en/function.get-browser.php#88923

In order for get-browser to work, your browscap configuration setting in php.ini must point to the correct location of the browscap.ini file on your system.  browscap.ini is not bundled with PHP.  It relies on user updates to keep the database current.