How to $_POST from a grid

I have set up a record grid, where the user may see a list of their "records". Repeat region, etc. There is a button next to each record the user may click to allow them to update the particular record. This all worked fine with $_GET where I did a link on the button to send the PrimaryID over. Now I'm attempting to use $_POST and not having the same success. I moved the grid into a form and have a submit button now for the "update" button. Since the grid only is echos of the recordset fields, how does it know the correct PrimaryID to send when I hit the submit button. I'm thinking this is perhaps my problem. Can anyone help please Stuart