Instead of using Radio buttons, use checkboxes.
name them all the same so that they will be a checkbox group.
the checkbox group will send the selections as an array. You can convert the array to a comma separated list using the following code on your page:
<?php
if($_SERVER["REQUEST_METHOD"] == "POST") {
foreach($_POST as $key => $val) {
if(is_array($_POST[$key])) $_POST[$key] = implode(", ", $_POST[$key]);
}
}
?>