<?php
function on_click($widget, $event, $data)
{
    switch($event->button) {
    case 1:
        /* do something appropriate to a left click */
        break;
    case 2:
        /* do something appropriate to a middle click */
        break;
    case 3:
        /* do something appropriate to a right click */
    break;
    }
}

$widget->connect('button-press-event', 'on_click', $data);
?>