dynamic checkboxes?

I have a form that generates a list of "categories" dynamically. That is, for each category in a database, the form has a corresponding checkbox. The checkboxes are given serialized names, such as "chkbx1", "chkbx2", etc. I'm having trouble figuring out how to process these in the php script that handles this form. In other words, how do I figure out which categories were selected when I don't know in advance how many categories there are? How can I check if the form variables "chkbx1", "chkbx2", etc have been set? Thanks, Matt