set the value of textbox using optionlist

Question

I have a database table named tblCityState with the following data:

CITY	STATE
New York	New York
Los Angeles	California
Atlanta	Georgia

on my aspx page I have a dropdown (ddCity) bound to the table (Select City from tblCityState).

When the user Selects "Los Angeles" from the DropDown, I want a textbox (tbState) to be populated with "California".

Now, I could set the value of the DropDown Item to be the State, but I want the Item value to be the City.

What is the best / easiest way to achive this with PHP?