PHP w/ FLASH security

Well…
Here is how it is right now. I have my flash movie communicating with my php pages. Now, how can i make sure that someone doesnt just type in the php pages url and go to it. Currently i make sure that they are posting the variables required, but I would like something more secure than that. It wouldnt be too hard to make their own form that used my scripts as an action. Understand what I mean… I tried session variables, but because of the way that they are communicating there are some errors. They just don’t seem to work… if someone has other experience tell me i am willing to see where i went wrong. I also tried verifying the refer page… and well it sees itself as a refer page because of the way flash is accesing it. So i guess I could use that… but is there anything you guys can think of?