I have a full website with two contexts for two different languages. The only public page is the landing page of both languages. The rest should be private/protected. This I have achieved with resource groups and limits on the anonymous users. On the landing page all the menu entries that are protected should be seen by the anonymous user and if clicked a popup with two login-forms should be displayed. These login-form are from other sites and will return if the users has permission or not when they've entered their credentials. And as long as this session exists the user should be able to view all pages if the user was approved of course.
My guess as a non modx- or php- pro is that I should check if a session exists when the landing page is loaded (and all sub-pages). If no user is logged in all links will point to the popup. The user then logs in, sends info to the external server and is redirected to the private/protected landing page if successful. And this is what I can't find any info about, probably because I'm not entirely sure what to look for. I need one snippet that checks if a valid session exists for the protected pages, if not display the logins. The other code I would need is something that creates the session for the user if the external login was successful. Should this be a snippet or just a php document on the server. And how can I start a session for the protected pages?
You could do this in two different ways:
Make a user-system that is not connected to Modx. I find this the easies and I've done this several times before. You'll need to make a table for users with usernames and password, and make an object out of it, so you can use xpdo to do the queries. With this system up and running, it would be no problem to include a snippet in every template to make sure the user is indeed logged in. If not, just redirect him to the correct frontpage/landingpage. This will require some coding, but as I said, it works like a charm.
Download the snippet http://modx.com/extras/package/login (by Spittingred, a true legend), and look at the code. I haven't used this Extra before, but I am pretty sure it uses the same user-system as Modx, and therefor you should be able to achieve what you want. I can't give you any more help than "look at the source and figure out how Spittingred did it".