xxxxxxxxxx
add_action( 'wp_ajax_foobar', 'my_ajax_foobar_handler' );
function my_ajax_foobar_handler() {
// Make your response and echo it.
// Don't forget to stop execution afterward.
wp_die();
}
xxxxxxxxxx
add_action( 'wp_ajax_my_action', 'my_action' );
add_action( 'wp_ajax_nopriv_my_action', 'my_action' );