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about how user agent detect,ua of browser

Main content page - about how user agent detect.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.
It is a request of sorts: the web browser is requesting access to the website. Part of this request, then, is that the web browser be identified as the user-agent. This identification of the user agent contains specific information, including the application name and version, the host operating system and language.
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.
Opera also offers a full masking as Internet Explorer or Firefox, which hides "Opera" completely. Beside browsers, other programs using the HTTP protocol, like most download managers and offline browsers, also had the ability to change the user agent string sent to servers to user's liking.
The user-agent string is one of the criteria by which web crawlers can be excluded from certain pages or parts of a website using the "Robots Exclusion Standard" (robots.txt). This allows webmasters who feel that certain parts of their website should not be included in the data gathered by a particular crawler, or that a particular crawler is using up too much bandwidth, to request that crawler not to visit those pages.