How to execute the same script after some period of time in php?

Question

PHP

For example, every 15 seconds we print ok.

Answer 1

To have the user in the browser? No way. We need javascript ajax, which will pull the php script every 15 seconds.
If you need php to perform some task, for example, clear a folder every hour, send emails every 24 hours, etc. without user intervention, it is possible. cron.

Answer 2

cron