PHP auto notification via email, the logic behind this

For example, I want my client to be notify via email on a set date and time in the future. Can PHP do this even if nobody is visiting the website?

What condition should I make? Should I save this on session, etc... I just want to know the logic on this. Any suggestions?

Thank you.

If the server is using Linux, then you can use cron jobs

You need to use cron job. And maybe its not the answer to your question but facebook uses this system too. Very interesting info about facebook's opensource platform https://developers.facebook.com/opensource/

You can do this if you use a cron (or equivalent) job with PHP. A cron job executes a page every x time.