jQuery autocomplete with IF statements

jQuery autocomplete with IF statements

Hopefully an easy question, I'm using jQuery autocomplete which is working
fine and is returning the values based on a SQL query where LIKE
$querystring.
Now I want to add an additional dropdown list on my ASPX page that will
run a different SQL query based on what is selected which will get passed
through to the Search.php at the same time the user is typing in the
autocomplete field.
So to put it simply I want to use the dropdown list to search different
rows in my SQL DB and return autocomplete results based on that row. Can
someone help me from the ASPX side and also how to read the value from the
PHP side.
ASPX code is like this http://jqueryui.com/autocomplete/#remote
Thanks