Adding Headers to a log file

I have a log file that I have created to in order examine how effectively my application is running. I have called this debug.log. What I am trying to do, is set it up so that each time the application is run the log file is updated by with the time and the particular user that is logged in at that time. Any ideas??????

When the user logs in, simply Append to the file the user name, and the "Now" statement

Code:

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printToLog (username)
printToLog (Now)

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