drupal_get_bootstrap_phase() calls drupal_bootstrap() without any parameters.
The default parameters of drupal_bootstrap cause the static $final_phase variable to be set to NULL.
If site bootstrap is still in progress when drupal_get_bootstrap_phase() is called this can result in a WSOD.
For example, if called from the cache backend during DRUPAL_BOOTSTRAP_VARIABLES phase the following error is produced:
PHP Fatal error: Call to undefined function menu_execute_active_handler()
Instead drupal_get_bootstrap_phase() should return the current bootstrap level without affecting the bootstrap process.
