Without knowing the structure of your site and information storage, there are several ways to do this:
Without going into all the gross details, I'll assume you are familiar with user authentication and validation schemes. The way I handle this problem is by use of sessions to store the user's username, or unique user ID number (generated by the auto-increment funciton of the mySQL database at registration). If $_SESSION[some_username_variable] is set (an indication that they were properly authenticated), you can use that to load their registration profile into a form.
Another similar method is to set several session variables (or a single session array) to store the $_POST information from your form. Upon submission do what you want with the data - store it or hold for verification . In any case, make sure the data is available to the script before you load the form so that you can properly design the form to display whatever data is available.
Hope this helps!
