Web Form: How can I use a user's selection to store related values into other hidden fields for saving?

I have a web form. Let’s say one field is ‘customer’, and when the user selects that value, I can automatically fill in several hidden fields with values from that selection’s record (like ‘address’ or ‘phone’, etc). How can I use PHP or EE to ‘grab’ that value for use in other input field tags, to store values at the time of the form submission?

The reason this is troublesome for me is because I’m using the EE tag ‘query’ to use SQL to pull the values I want in the drop-down list for the user to select, so when they do, I can store the proper value into THAT field with no problem. The problem is getting the OTHER info out of that input segment for use in the hidden input segments. Maybe a PHP variable or something?

Let me know if I need to be more clear. This one is bugging me…

-Tony