generate a log file with site statistics

I have a PHPBB based forum where I'd like to automatically generate a log file with some site statistics.
every time there is a hit on a topic page (which runs a file named viewtopic.php),
  I want to generate a new line in this log file with the following info
    Date/Time, IP address, topic url  
  For the sake of example, let's asume the variable names for these items are date_time, ip_address and topic_url
This seems simple enough but I'm still new with PHP, I would appreciate some direction. Thanks

Thanks