title: What and Where comes the actual usage of LARAVEL_START const defined in index.php of Laravel 5.7?
body: <p>When I'm looking into the Laravel Request Cycle, I encountered this line of code in the file <code>index.php</code></p>
  
  <pre><code>define('LARAVEL_START', microtime(true)); 
  </code></pre>
  
  <p>I didn't find any specification about this in Laravel documentation, may be it's not Laravel specific, who knows. Also I can't find any usage of this either in the documentation or any other place, please help me to understand what this line of code actually for?</p>