SSI.php is included in SMF. To use it, you have to use these commands on the main page of your website:
<?php include('http://www.example.com/SSI.php?ssi_function=recentTopics'); ?>
<?php include('http://www.example.com/SSI.php?ssi_function=topPoster');?>
<?php include('http://www.example.com/SSI.php?ssi_function=whosOnline');?>
Please see ssi_examples.php, located on your forum's root folder, for more examples.
For more information on SSI.php, take a look at Using SSI.php.
