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user agent string lookup and checker

Main content page - user agent string lookup and checker.Like the user-agent string for Internet Explorer, this user-agent header provides details about your system. Alternate tokens can appear in the user-agent string for a number of reasons; usually, they identify optional features installed on your system.
This topic describes the user-agent string, which identifies your browser and provides certain system details to servers hosting the Web sites you visit. The topic also shows how to view your user-agent string, summarizes tokens used by recent versions of Windows Internet Explorer, and documents registry keys that affect the user-agent string.
Today, nearly everyone uses a web browser as their user agent. Sometimes it's necessary for a web site to understand how it is being viewed, so most user agents identify themselves by sending a User Agent String to the web site.
The user-agent header identifies the application requesting the information from the server. The user-agent string can contain optional details called tokens, which must be enclosed in parentheses and vary among programs.
In short, it cannot really be trusted. 'HTTP_USER_AGENT' Contents of the User-Agent: header from the current request, if there is one. This is a string denoting the user agent being which is accessing the page.
The Web server can use this information to provide content that is tailored for your specific browser. Because certain non-Microsoft sites add additional details to the user-agent string, it is important to understand the user-agent string.