documentation/magento2-base_2index_8php_source.html

 <?php
 try {
     require __DIR__ . '/app/bootstrap.php';
 } catch (\Exception $e) {
     echo <<<HTML
 <div style="font:12px/1.35em arial, helvetica, sans-serif;">
     <div style="margin:0 0 25px 0; border-bottom:1px solid #ccc;">
         <h3 style="margin:0;font-size:1.7em;font-weight:normal;text-transform:none;text-align:left;color:#2f2f2f;">
         Autoload error</h3>
     </div>
     <p>{$e->getMessage()}</p>
 </div>
 HTML;
     exit(1);
 }

 $bootstrap = \Magento\Framework\App\Bootstrap::create(BP, $_SERVER);
 $app = $bootstrap->createApplication(\Magento\Framework\App\Http::class);
 $bootstrap->run($app);
Magento\Framework\App\Bootstrap\create
static create($rootDir, array $initParams, ObjectManagerFactory $factory=null)
Definition: Bootstrap.php:119

Magento\Framework\App

__DIR__
defined('TESTS_BP')||define('TESTS_BP' __DIR__
Definition: _bootstrap.php:60

exit
exit
Definition: redirect.phtml:12

Magento

BP
const BP
Definition: autoload.php:14

$app
$app
Definition: index.php:38

$bootstrap
catch(\Exception $e) $bootstrap
Definition: index.php:36