Performing an action in the Mysql DB without user reloading?

Hi, I’m pretty new to PHP, and I was wondering how exactly the following works:

If I set a certain time in a PHP file, and the current time reaches that time, then it will perform an action with the MySQL Database automatically without requiring a user to load the page at that certain time. Because my problem is that every script I put in requires a manual reloading of the page for it to actually execute.

How will I be able to do this?