The title and description of this topic should tell you my basic question, but it's never enough detail just to read a question, so I will explain my intention.

Basically, on my website I have a php script for users to register to my site. I then use this data to create forum accounts (I do not use the main forum Registration form because I cannot change it's contents and there are some questions I'd like to ask before registration). Later I also plan to make hidden areas that you need to register for to enter.

Anyway, I wondered if it was possible to make a login page where you could choose to be logged in to the site or to our SMF forums, and if you chose the forums, you would instantly get logged in to the forums without having to type your data again. Do you know any way to do this?

Think "spash page" and "cookies".
The splash page to input the data which I assume you would store in a database.
Then cookies to set the forum required info.
Most forums set their own cookies already, so check to see if the cookie is present before setting another one.
In the spash page, direct the user to either an html page if NOT forum is selected or to the forum if selected.
Using the Cookies will also let them enter the forum from their Favourites/Bookmarks list on their local browser.

uhm.. he is looking for script, not explanation how to build that in php
anyway i think that IPB have much much more hacks and mods that you can use.
So im not sure if there is a script for that you are looking for on SMF forums, i know there is on IPB.
i sugesst you to change on IPB... but its your choice.

I will think about changing from SMF to IPB, but I do not want to resort to using a forum host, as I have said in countless other topics, so that may be out of the question.

Jlhaslip has actually explained quite a good method for me. I will endeavour to use it.