howto restrict reaching functions from url

[eluser]emrah[/eluser]
This worked like a charm. I'm really grateful.

Thanks for your advice also.
This will be a local site in a university and users are the IT personels.
It is like a small crm which holds the inventory data and prints barcode for.
And also service records for the inventories will be holded.
So it needs to have a basic interface which users can interact easily like a win32 application.

That's why javascript is a "must have" for me.
I don't want users get lost in the pages )

Do you have any other suggestions about security because of using ajax?