Session tracking and multiple form updates

Hello, I am working on a web site that is available only over HTTPS and am using session tracking. So far I am only saving the person's display name (different then login), site role and e-mail address in the "$_SESSION" data. The rest of the site is dynamic, now the question is on three particular pages the user fills in a form and hit's submit. Now at the moment on each page is a hidden variable containing a db primary key for which the data is getting updated. I know that hidden values are not a good idea since the user can change them ... so I was going to store the hidden value in a "$_SESSION" variable but then the problem is ... what happens when the user has two - three windows open and they are updating two - three different items. Each having it's own db key ? If I stored the key in a session variable the last one opened would over write any previous one. I hope I explained this correctly ... multiple updates and access to the site is allowed and can not be changed. Thanks. Michael.