passing variables from one page to another

I'm writing a trouble ticket system I have a login page (and sessions) which print links to various parts of the sytem on successful login. On another page (also controlled by the session), I have a form to check a serial number and print the user info if the serial is good. At that point, the user (a tech support person) has the option to create a new trouble ticket or edit an old one. What I am wondering is this: Is there a way to pass the serial (that was used for the search) as part of the update.php or new_trouble_ticket.php forms (like in a hidden field or something) ? I want to avoid having to enter the number twice during the same call, because the serial number is very long and it's already associated with the client's contact info. If the tech entered it wrong, it would screw things up in a big way. In clearer terms, What I'm looking to do is make it so the serial number is entered once (once meaning just during each individual tech support call), at the time that the tech support agent enters it when a user calls for support. They type in the number, and either it's good or not. If it will help, I will post the source code, but it's long, and I don't want to clog up the group any more than neccessary. Sean I'm going to feel like an idiot if the soulution is right under my nose.