title: PHP daylight saving time detection
body: <p>I need to send an email to users based wherever in the world at 9:00 am local time. The server is in the UK. What I can do is set up a time difference between each user and the server's time, which would then perfectly work if DST didn't exist.</p>
  
  <p>Here's an example to illustrate it:</p>
  
  <ul>
  <li>John works in New York, -5 hours from the server (UK) time</li>
  <li>Richard works in London, UK, so 0 hour difference with the server.</li>
  </ul>
  
  <p>When the server goes from GMT to GMT +1 (BST) at 2:00am on a certain Sunday, this means that John now has a -6H time difference now.</p>
  
  <p>This scenario I can still handle by updating all the users outside the server's local time, but once I've moved forward/backward the time of all the other users, I still need a way to detect <em>when</em> (time and date) the users living outside the UK will (or will not) change their local time to a probable DST one.</p>
  
  <p>I need a PHP method to know/detect when other parts of the world will enter/exit DST.</p>