Restrict Record Access via the web

I have a database that I am accessing by different User groups via php. This was previously an IWP solution. I have about 20 different privilege sets which restrict access for Users to the appropriate records. In IWP, the record access was prompted when the user logged in, a script was run to find only those records. All finds were controlled by buttons restricting access to those records. The privilege sets are named for the User's "site" so the previous IWP script would get the privilege set from a field value (Get(privilegeset)) and always constrain any find with that value.

Is there a way to mimic those finds in PHP code? For example, if I login as one of the restricted access user, I see all the records in the database, but a large percentage are blank because the user does not access to view those records. If I go to the search page, I can only choose the User's site from the site pop-up list (even though there are 11 sites), which then will return the correct set of records. I suppose I can train my Users to always perform a find for their site before doing anything, but I am wondering if I can store it in the code of the page and parse it from page to page, without the User knowing.

I am open to any suggestions. Currently, I just have a system (with many related tables), in which Users have access to their records only. I want to make this seamless over the web, if possible.

Let me know if more information is needed. Thanks!