inactive

You need to look at a solution that will move the actual market, not re-load the whole map area. Load the updated lat/lon via json, then reset the marker position.

It's called casting, i.e. as above, converting from one type to another. In most cases not necessary with PHP, as it implicitly converts when necessary.

Is that two return statements in isLoggedIn? The second one will never be executed.

You could do somedomain.com/clientname and use mod_rewrite (via .htaccess) or some other "friendly" url scheme to push all request to one file, which you use to look up the client's details in the db. Also consider looking into WordPress, which has multisite functionality, and auto sign-up options as well.