Symfony2 dynamic form fields

What you're looking for is AJAX. This can be implemented with straight javascript or jQuery (jQuery is much cleaner and easier to use).

Here's a jQuery stub for you to get an idea of what has to happen:

<script language="javascript">
    $('#mySelect').change(function() {
        $.post('backgroundScript.php', {
            val: $(this).val()
        }, function(data) {
            $('#radioDiv').html(data);
        });
    });
</script>

If you're not familiar with AJAX & jQuery, I'll explain what's going on. $('#mySelect') is almost the equivalent of javascript's getElementById() function. They are NOT the same functionality (jQuery uses the same notations as css to find the elements you're looking for), but this will return the element with mySelect as its id attribute.

.change() sets the onChange event handler to the function() defined within the brackets. $.post is jQuery shorthand for performing an AJAX post (as opposed to a get), that is sent to backgroundScript.php without a page load. When backgroundScript.php returns with a 200 status (i.e. "everything's OK, here's my output") it will perform the callback function function(data).

backgroundScript.php will receive val as a $_POST variable and do "something" to determine which radio buttons should be displayed, then will write output to the screen.

EDIT (more info):

backgroundScript.php should look something like this if you're determining the radio buttons based on a database. Keep in mind that this is not written with security or consideration of deprecated functions in mind (e.g. mysql_*), just functionality:

<?php
// backgroundScript.php
$masterVal = $_POST['val'];
$output = "";

$query = "SELECT bt.button_value, bt.button_label
    FROM button_table as bt
    WHERE bt.button_master_id = " . $masterVal;

$result = mysql_query($query);

while($row = mysql_fetch_array($result)) {
    $output .= "<input type='radio' name='radioGroup' value='" . 
        $row['button_value'] . "' /> " . $row['button_label'];
}

echo $output;