Create the radio buttons using new GtkRadioButton(buttongrp, button_label). Note that for the first radio button, we pass in NULL as the first argument. For the second radio buttons, we pass in the pointer of the first radio button as the first argument. Similarly for the third. PHP-GTK will then group all these into the same group.
Connect the signal 'toggled' to the function on_toggle().
Note the first argument $radio. This is passed along by php-gtk when calling your signal handler.
Get the label of the button that is being clicked on.