Send email to admin when an appointment is cancelled

Send email to admin when an appointment is cancelled.php

<?php
function my_app_email_on_appointment_cancelled ($app_id) {
	global $appointments;
	$to = $appointments->get_admin_email();
	wp_mail($to, 'An appointment has been cancelled', "An appointment with ID {$app_id} has just been cancelled.");
}
add_action('app-appointments-appointment_cancelled', 'my_app_email_on_appointment_cancelled');