Authenticating through a php script

I am working on a script that will authenticate on a central system my company has devised for us to use (LDAP) and then authenticate them to other sites that I want them to access (Online Databases and other electronic resources I do not control but pay lots of money for all students to access). I have seen this done on a product produced by Epixtech called RPA (Remote Patron Authentication). This is an authentication system that avoids using a proxy server. It basically handles the authentication (LDAP) and sends the appropriate information to the other secure source (Online Database, Electronic Resources, or my online catalog's patron information.) Typically there are multiple ways it will authenticate for the user to other resources. URL referer, ip authentication, fill in an user/password form for the user. I just can't get the user/password portion to work on a protected site. My tests of sending post information to another one of my scripts works fine. But it doesn't work as of yet. I have worked a bit with scripts that send post information through sendToHost function (fsockopen and fputs). But nothing is really working here. Does anyone know how I should go about this? All suggestions will be great! Thanks a bunch, Jeremy