title: PHP : Delay instructions with CLI or Crontab and NOT Sleep()
body: <p><strong>Context</strong><br>
  I need to warn my website's users of events by sending them an email when they're offline or display them a javascript notification if they're online (using websockets/push events).<br>
  Some of these events should be triggered <strong>after a certain amount of time</strong> (for specific app/business logic).</p>
  
  <p>I won't use <em>sleep()</em> since it may causes performances issues.</p>
  
  <p>How would you manage to do that using Crontab and/or CLI  ?</p>