Automatic form initialization in PHP

Hi, Could someone give me a hint or pointer to a tutorial on how to do the following? I would like to set-up a web page to update the entries in a database of people. I´d like to reuse the submission form I use to register a new user where all the personal information is requested (telephone, office, name ,etc.). Assuming I have all the fields corresponding to the user whose info has to be updated, how should I pass this information to the submission form so that the (already registered) user only has to modify the information that has changed? I´m working with PHP. I would appreciate a pointer to an example code doing this. Cheers, Silvia