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.

Recently I experimented a bit with an Apache log file analyzer written in PHP. It's not all that difficult were it not for trying to parse the browser, or user agent, string. [...] For my purposes I don't care much for the operating system details. This is the result so far. I'm still not very satisfied but I thought maybe other people might be interested and maybe help out.

He provides the code in the format of an encapsulated PHP function that returns the product they're using, the version number of the browser and other various comments.