title: Obfuscating database ID to customer facing number
body: <p>I'm using mysql database auto-increment as an order ID. When I display the order ID to the user, I want to somehow mask/obfuscate it.</p>
  
  <p><strong>Why?</strong></p>
  
  <ul>
  <li>So at first glance, it is obvious to admin users what the number
  refers to (orders start with 10, customers start with 20 etc)</li>
  <li>To hide, at first glance, that this is only my 4th order.</li>
  </ul>
  
  <p>Based on this <a href="https://stackoverflow.com/a/178589/1587851">this answer</a>, I want the masked/obfuscated order id to:</p>
  
  <ul>
  <li>Be only numbers</li>
  <li>Consistent length (if possible)</li>
  <li>Not cause collisions</li>
  <li>Be reversible so I can decode it and get the original ID</li>
  </ul>
  
  <p>How would I acheive this in PHP? It doesn't have to be very complex, just so at first glance it's not obvious.</p>