i am making a web site that is based on online examination in that i have pro is that how can i imlement the time when a student click on start button after his login.

You need to use session to store the starting time of the Exam. Also you need to use javascript timer(may be ajax), store the starting time in a javascirpt variable and then check whether the exam time is elapsed at client side. You may need to check the elapsed time at server side whenever the user post back to the server.

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