Forum OpenACS Q&A: Re: How to force login for the Site?
Sounds like what you want is a single integrated log-in across your OpenACS and PHP applications, but I don't think anyone's going to be able to help very much unless you tell us what those PHP applications are, how they (or if) they are currently attempting to do their own stand-alone login/authentication, etc.
On the other hand, if PHP application isn't trying to do much of anything with login or authentication on its own, then probably all you really want is to force an OpenACS login whenever the client requests certain classes of URLs. You should be able to do that with an AOLserver registered filter on all those URLs calling ad_maybe_redirect_for_registration or some similar OpenACS login proc. There might be a better way, perhaps by fiddling with the request processor, but the registered filter route should work.