Need HOWTO on field-update code.

Can PHP update a second field on the form, based on the contents chosen from the first field (I'm thinking of JavaScript's function) ? I'm pulling a list of id,name,email from MySQL and displaying name for the first field. After the user picks it, I want the appropriate email address to show up in the second field. 1. I want to do this without sending a second select request to the server. 2. What PHP functions should I be looking at to build this? 3. If not PHP, where can I find the appropriate JavaScript examples? Thanks, Eman