This is my answer to a Question on Support site for LampCMS project
Yes, you can run multiple sites from single Lampcms library, it even comes with a lampcms.phar - php archive where entire library is packages in one file.
Every site will need own www directory with all the templates, css files, js files and own config file but the lampcms engine - can be shared between sites.
If you going to use sub-domains like you mentioned, then I'm pretty sure the you can also share database of users and user logged-in to any of your sub-domains will also be able to use any other sub-domain.
I have never done this, but in theory you can share user collection in MongoDB between multiple sub-domains in order to implement a single login.
If you run into any issues when setting this up, just ask for help here, I will help you.