title: How to execute code in linux every 500 miliseconds?
body: <p>I have an http server on Linux.<br>
  I have some PHP code that refreshes my cache and I would like to execute this code every 500 milliseconds whenever the server is running, even after a server restart.<br>
  I was thinking of implementing a service but I don't have much experience with Linux so I don't know how to do this or if it is even the right approach.<br>
  I also heard of cron-jobs but I see that they run by the minutes and not by the seconds.<br>
  How can I execute PHP code in Linux every 500 milliseconds(Code would be helpful)?</p>