什么和哪里来了Laravel 5.7的index.php中定义的LARAVEL_START const的实际用法?

When I'm looking into the Laravel Request Cycle, I encountered this line of code in the file index.php

define('LARAVEL_START', microtime(true)); 

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?

define('LARAVEL_START', microtime(true));

This line essentialy starts up the timer, so you can time how long it takes to boot up the framework, etc. Fun fact is that this constant is never used throughout entire framework, but you already noticed it.