drupal/core/install.php

<?php

/**
 * @file
 * Initiates a browser-based installation of Drupal.
 */

use Drupal\Component\Utility\OpCodeCache;

// Change the directory to the Drupal root.
chdir('..');
// Store the Drupal root path.
$root_path = realpath('');

/**
 * Global flag to indicate the site is in installation mode.
 *
 * The constant is defined using define() instead of const so that PHP
 * versions prior to 5.3 can display proper PHP requirements instead of causing
 * a fatal error.
 */
define('MAINTENANCE_MODE', 'install');

// Exit early if running an incompatible PHP version to avoid fatal errors.
// The minimum version is specified explicitly, as DRUPAL_MINIMUM_PHP is not
// yet available. It is defined in bootstrap.inc, but it is not possible to
// load that file yet as it would cause a fatal error on older versions of PHP.
if (version_compare(PHP_VERSION, '7.3.0') < 0) {
  print 'Your PHP installation is too old. Drupal requires at least PHP 7.3.0. See the <a href="https://www.drupal.org/requirements">system requirements</a> page for more information.';
  exit;
}

// Initialize the autoloader.
$class_loader = require_once $root_path . '/autoload.php';

// If OPCache is in use, ensure opcache.save_comments is enabled.
if (OpCodeCache::isEnabled() && !ini_get('opcache.save_comments')) {
  print 'Systems with OPcache installed must have <a href="http://php.net/manual/opcache.configuration.php#ini.opcache.save-comments">opcache.save_comments</a> enabled.';
  exit();
}

// Start the installer.
require_once $root_path . '/core/includes/install.core.inc';
install_drupal($class_loader);