I want to schedule sending of email from php script. I want user to specify date and time and then on the specified date and time, I want those emails to be sent automatical
PHP usually uses an external scheduler for this sort of thing. That means cron on *nix or Windows Task Scheduler on Windows.
If you want to set it up through a web interface, then you might consider storing your schedule in a database and having cron (etc) kick off a script that looks for overdue emails every 5 minutes.