get_browser() attempts to determine the capabilities of the user's browser. This is done by looking up the browser's information in the browscap.ini file. By default, the value of $HTTP_USER_AGENT is used; however, you can alter this (i.e., look up another browser's info) by passing the optional user_agent parameter to get_browser().
While browscap.ini contains information on many browsers, it relies on user updates to keep the database current. The format of the file is fairly self-explanatory.
The following example shows how one might list all available information retrieved about the user's browser.
The output of the above script would look something like this:
In order for this to work, your browscap configuration file setting must point to the correct location of the browscap.ini file.
For more information (including locations from which you may obtain a browscap.ini file), check the PHP FAQ at http://www.php.net/FAQ.php.