Is there any way to do this in PHP (or with a combination of JS)?

[immediate]

There's a login form on page A: example.com/A.html

The 'action' attribute of which is: action="/B.html"

So when someone logs in with the form on A.html, the data will be submitted to / visitor redirected to B.html.

Now what I want is: there's an <iframe></iframe> on /C.html, framing B.html, is there any way so that when someone logs in with the form on A.html, the visitor is redirected to C.html and the login details is submitted to B.html that's framed on C.html in the <iframe></iframe>? So after the person logs in from A.html, he arrives at C.html with the actual CP dashboard framed in an <iframe>.

I have absolutely no clue in achieving this. Maybe the form will be submitted to C.html first and C.html submits the login details to B.html (framed in C.html) via JavaScript? How?