load list of values to the listbox based on another listbox value

The basic idea is to submit that data to the server (either by POST back, or AJAX), and then respond with the data.

<select id="mySel" onChange="sendData()">

What I've done there is added a javascript function to be called every time the drop down value has changed.

function sendData() {
    $.post("processData.php", {selected: $(this).val()}, updateData(data));
}

This is a skeleton of the function I'd write for the select onChange event. I've skipped a step or two here and used jQuery to help create an AJAX request back to the server. I will be calling my php script processData.php to help process which element was selected. The {} contain the data I want to send to the server, in this case the selected value. And Finally what to do once I get data back from the server.

Now I'd be in my php file and able to process the data I took in and run my query to get the new data. Once done I simply json_encode the data and respond with it.

Now back in the javascript world my UpdateData function is automatically called and passed the json data.

function updateData(data) {
    var select = '<select name="sel2">';
    $().each(data, function(index, val){
        select += '<option name="'+ index+ '">'+ value+ '</option>';
    });
    $("#mySel").parent.append(select);
}

That would allow me to generate a new select list from the returned data (assuming a key/value paired array in json).

I haven't actually tested any code and it's designed to be more of a guide and pseudo-code.