PHP language doubt in fatching the values in 2nd list box

I would advise to use jQuery for processing such events. Please see:
http://en.wikipedia.org/wiki/JQuery[^],
http://jquery.com/[^],
http://docs.jquery.com/Main_Page[^].

Moreover, you can get something more advanced which is developed with some advanced handling in mind. Look at this widget, jqxListBox:
http://www.jqwidgets.com/jquery-widgets-documentation/documentation/jqxlistbox/jquery-listbox-getting-started.htm[^].

It helps you to define the list box and handle selection, in particular.

This is a part of jQWidgets suite. http://www.jqwidgets.com/[^].

However, check the license, especially if you are working for commercial use: http://www.jqwidgets.com/license/[^].

If you don't want it, it can develop your own jQuery plug-in of find something else.

—SA