fix php error - undefined variable
do not allow the client to edit appointments that occur within 24 hours
added a command line interface for sending appointment reminder emails

the function sends an email to appointments that occur within the next 24 hour peroid (if they have not yet been notified). to send reminders the database schema must first be updated:

php index.php cli update

after that, a cron job can be set up to run the following command:

php index.php cli send_appointment_reminders