title: Is a timestamp in microseconds always unique?
body: <p>uniqid() in PHP generates a unique ID based on the current timestamp in microseconds. Is that really a foolproof way to generate a unique ID?</p>
  
  <p>Even assuming there's a single user running a single script with a loop generating a timestamp in microseconds, can there still really be a theoretical guarantee that it's unqiue? And in practice, is the likelihood completely negligible?</p>
  
  <p>For clarity, say your loop is nothing more than this:</p>
  
  <pre><code>foreach($things as $thing){
      var_dump(microtime());
  }
  </code></pre>
  
  <p>is there any theoretical chance it might not be unique and, if so, how realistic is it in practice?</p>