backdrop_bootstrap in core/includes/bootstrap.inc

Ensures Backdrop is bootstrapped to the specified phase.

drupal.inc in core/includes/drupal.inc

Contains constants and function wrappers for Drupal 7.x compatibility.

http.php in core/modules/simpletest/tests/http.php

Fake an HTTP request, for use during testing.

https.php in core/modules/simpletest/tests/https.php

Fake an HTTPS request, for use during testing.

install_begin_request in core/includes/install.core.inc

Begins an installation request, modifying the installation state as needed.

update_prepare_bootstrap in core/includes/update.inc

Performs extra steps required to bootstrap when using a Drupal 7 database.