need to get php code that's on a page on my joomla website (a registration form page)?

[Emad Saeed]

Hello, I'm a bit new with joomla, so there's this registration form that I'm enhancing its design on my friend joomla site, so I put a new html page with the same name of the registration page (to replace the one formed by joomla itself, & there was no problem with that), but I think there's some kind of php on the form code, so how can I get that code, like where can I find it in the joomla adminstrator panel or even the template for it anywhere over the internet?
thanks alot, I really appreciate it.