Did the OP get there solution? To use the parameter from the GET request you would probably have to encode and store the full relative URL and parse it that way. Which could present problems depending on browser security.
I cannot help but think the OPs request wasnt to load a website in an iframe but to try to engage and dabble in some XSS or other nefarious things. Loading a php page in an iframe is easy. Trying to use parameters and arguments is a different kettle of PHISH.