Modify url appearance without redirecting to another page or folder .htaccess

I have a url mysite.com/subscribe.php when someone goes to that url, in the url window I want the subscriber to just see mysite.com/passthrough. I do not want to redirect them to mysite.com/passthrough I still want to utilize all of the functionality in subscribe.php but without showing the script name to the subscriber. I will also be passing various POST variables to that script as well.

I will never be sending anyone directly to the url mysite.com/passthrough, that folder does not exist on my site, I simply want to mask subscribe.php so no one can see the script that I am using.

I've scoured the web for hours trying to figure out how to do this, any help would be extremely appreciated.