I have simple php billing application where the admin can enter his expense report(with due date and paid option with check box)
How to remember him with automatic mail if the payment is not done (if he did the payment he can edit his report) and it is overdue with php ?
Its very simple for automatic tasks, in linux based servers we used Cron Jobs.
Its a simple task that run on time interval you defined and run php file you specified in cron job.
So create a cron job with once a day, and give path to your php script that will check in database if any record with no payment and send email.