php getUrl function

is their a function in php that can be used to get the whole URL from the previous page?

For Example:
if I have a link clicked that goes to http://www.mydomain.com/main.php?var=...

How can I get that URL with a function, I'm particularly interested in the part main.php?var=...

There is a server global: $_SERVER['HTTP_REFERER']....but it can;t be relied upon as accurate...

I would use SESSIONS and send the previous page's URL to the next page that way...