There are numerous ways you could implement this in PHP, but it would at the very least require writing a short script. (In any language I imagine?) HTML form -> Process -> Maintain state (in cookie/session).
Does classic ASP really have something like this built-in?!
I think I know what you are talking about - this is often used on login pages, where, if you want to access certain page, you are sent to login page, and after you log in, you are then sent to the page you tried to visit.
I did not realise this is inbuilt function in ASP and .NET, but from the "outside", what I am seeing in URL of the (for example login script) is something like:
From the login.aspx page, the POST is executed to the above URL (to the login.aspx script), which, after checking the credentials are ok, redirects to www.example.com/some-target-page.aspx specified within the returnURL parameter
I am not aware if this exists as a function in PHP, but it can be coded so that if you get returnURL (or similar named parameter), your script performs a redirect to a location as defined in the parameter if the credentials are ok.