Every time you use your browser to access a website a User-Agent header is sent to the respective server. Detecting user agents on the server can be useful for many reasons: browser quirks, personalize content, preventing illegal access.

He talks about the get_browser function that's included in PHP but that requires a browscap.ini file to work. His other option is the Net_UserAgent_Detect PEAR package. It grabs the user agent and breaks it up into the browser type, operating system information and any Javascript-related headers that come along with it. There's also useful tests like "isIE()" and "isNetscape()" built into the package.