title: Unique id consisting of only numbers
body: <p>I need to get <strong>time</strong> based <em>unique</em> id which would only consist of numbers, i was thinking to simply use something like this:</p>
  
  <pre><code>str_replace(".", "", microtime(true))
  </code></pre>
  
  <p>I would use this id for many things, for example comments posting, and i'm expecting quite high traffic so i would like to know how high are the chances of collision with <code>microtime</code> function?</p>
  
  <p>And perhaps there is a better way to get numerical unique id? Just remember that it has to based on time so sorting could be done.</p>