PHP forms- many MySQL queries

[Kostas Tsirigos]

Hello everyone! First of all, thanks to all of you who replied to my last post concerning drop-down menus. I finally got it going!!
Here's another problem:

When the user is presented with a form and he can search in several fields of a MySQL database, how can I combine the information the user supplies in the form? And how about when one or more cells are left blank because the user isn't interested in searching in those fields?

I really need some hint from where to start. When the user input is just one field, the SLQ query is easy, but the above problem I find it difficult to tackle.

Thanks in advance!