Access to Private Pages Using SSI.PHP

[chemdata]

Hi.  I wanted to use SMF to control all username and passwords on my website and to also to serve as the forum.  I see that I could use ssi.php to capture a lot of information on to many of the pages of my website.  Since I have both public and private(members only)  pages I wanted to know how that could be accomplished without having to resort to having members having to enter usernames and passwords twice.  I thought of possibly using the ssi.php feature to see if a person were signed on to view private pages and didn't know if this was feasible and if it was, how to do it.  Thanks for any help.