I'm running into a problem that I can solve fine by just submitting a form and calling a db to retrieve/echo the information, but AJAX seems to be a bit different for doing this (and is what I need). Earlier in a form process I ask for the zip code like so:

<input type="text" maxlength="5" size="5" id="zip" />

Then I have a button to continue, but this button just runs a javascript function that shows the rest of the form. When the rest of the form shows, I want to pre-populate the City input with their city, and pre-populate the State dropdown with their state. I figured I would have to find a way to set city/state to variables, and echo the variables into the form. But I can't figure out how to get/set those variables with AJAX as opposed to a form submit. Here's how I did it without ajax:

$zip = mysql_real_escape_string($_POST['zip']);
$q = " SELECT city FROM citystatezip WHERE zip = $zip"; 
$r = mysql_query($q); 
$row = mysql_fetch_assoc($r); 
$city = $row['city'];

Can anybody help me out with using AJAX to set these variables? Thanks!