I believe what you're trying to accomplish could be done by crafting a POST request to the zenphoto login URL, putting the username and password as POST fields, but not in the URL itself. Adding credentials to the URL wouldn't be seen by zenphoto, since it doesn't expect them there (and rightly so!) - that is a big security risk. Just imagine Google caching a link containing a username/password, or someone bookmarking the page and someone else being able to login as them from that bookmark.
So I think a reasonable solution would be to generate a link not containing the credentials, but that executes a script in fengoffice which creates a POST request (a form submission) setting all the same variables as the zenphoto login page, sending this to the zenphoto lgin URL (which simulates clicking "Submit" / "Login" from the form itself), and then forwarding the user to zenphoto once the response is received.