Okay, I need some serious programming answers PLEASE!!!!. Here is the scenario:

Customer wants users to authenticate based on where they came from. They have several locations that the users will be coming from. They don't want anyone to be able to access their website from anywhere other than these locations. The locations ip addresses will be changing regularly. Is there a way to have a page on the INTRANET internally that the users will go to and it will start a session or place a cookie and pass them to the website. The website then looks for that session or cookie and lets them in or denies them based on the session or cookie. The sites that they will be coming from are ASP and .NET servers and it encrypts the URL that it is coming from. The website it is going to is on a PHP server and is built on PHP and MySQL. I have asked this in like every forum on the internet I can find and no one seems to have a solution. Any help would be greatly appreciated.

You are best (i think) by using session cookies. (it's a mix) to find complete and accurate PHP scripts check here:

www.w3schools.com/php

Thank you for responding so quickly!!

Will I be able to pass the cookie from the ASP or .NET server to the PHP machine?

There is another option: ASP.Net, it is one whole thing. It used to be called ASP+. Check out below for more details. The cookie will go through PHP and then to Mysql, if you have Mysql installed. Which you should have if you are working on a remote server.

www.w3schools.com

Hm, if you want to pass a Session from one .NET page to the another .NET page in C# do this =

void Page_Load(Object sender, EventArgs e)
{
Session["user"] = "authenticated"; // change this to whatever you want to recongnise them
}

Thank automatically starts a session, and the next page you can use it to see if they are allowed in or not.

Your situation is very very strange.

Your clients reside on a .NET server. (???)
Your server is PHP / MySQL... (???)

Do you mean that your clients use windows IE browser?
If so, the browser will not start the session, your PHP App would init a session when the user visits your PHP App. Let you visitors directly access your PHP app via the browser.

You want to pass a session cookie from a .NET server to a PHP server.
It is equally strange.
IMO, it is not possible. If there is a such thing, I am sorry that I am not yet aware of it.
A .NET session is different and a PHP session is a different world.

When you want to make both worlds talk, you need to use XML (SOAP) format to make both your apps understand what you mean.