Welcome to the 5th installment of the Drupal 7 Line by Line series of articles.
In the first four installments I covered index.php, the basics of the drupal_bootstrap() function and the first two phases of the Drupal bootstrap process: DRUPAL_BOOTSTRAP_CONFIGURATION and DRUPAL_BOOTSTRAP_PAGE_CACHE. (There are links to those articles found at the bottom of this article if you want to catch up.)
In the previous article about the page cache bootstrap process I noted that unless your site is configured in a specific way, both the DRUPAL_BOOTSTRAP_DATABASE (phase 3) and DRUPAL_BOOTSTRAP_VARIABLES (phase 4) bootstrapping phases will be completed before the page cache (phase2) bootstrap process can itself finish. As a result, I am technically covering the database bootstrapping process out of order and am not literally following the line by line code execution of Drupal exactly. I hope you can forgive me.
The third Drupal bootstrap phase begins with a call to the _drupal_bootstrap_database() function.
The first few lines of this function can put to rest any argument that Drupal is an application development framework (like Zend framework, or CakePHP, or symfony). In its current state it simply isn't. Here, on virtually every page load, Drupal checks to see if it in an installation state.