Leaving settings.php in place (but writeable), per instructions 13 + 14 of major upgrade section of https://api.drupal.org/api/drupal/UPGRADE.txt/7.x , with the new D7 codebase but the old D6 DB, i get the following errors:
Notice: Undefined offset: 0 in Database::parseConnectionInfo() (line 1529 of [...]includes/database/database.inc).
Notice: Undefined index: driver in Database::openConnection() (line 1681 of [...]includes/database/database.inc).
Notice: Undefined index: driver in Database::openConnection() (line 1681 of [...]/includes/database/database.inc).
DatabaseDriverNotSpecifiedException: Driver not specified for this database connection: default in Database::openConnection() (line 1682 of [...]includes/database/database.inc).
Notice: Undefined index: driver in Database::openConnection() (line 1681 of [...]includes/database/database.inc).
The website encountered an unexpected error. Please try again later.
Here is the core of the question -- according to the instructions, and normal upgrades, update.php should note that settings.php is D6-style, and change it to D7-style, including db-connection format.
It is not doing so. What could be going wrong? Reading through the script, it determines the DB schema... but it's not obvious to me where it determines the settings.php version and updates it. Help? Thanks!
The installed Drupal 6 release is 6.38, the latest release for Drupal 6.
I first tried to upgrade to Drupal 7.95; then I tried to upgrade to Drupal 7.34, and then again with Drupal 7.0, but I had the same issue.
[[Note: the below is not part of the core of this question, but responds to "why don't you just hardcode D7-style DB connection? I can hard-code the database settings using the Drupal 7 array structure, but that leads to other problems (expecting things that don't exist yet):
Error
Error message
PDOException: SQLSTATE[42S02]: Base table or view not found: 1146 Table 'drupal_7_test.main_system' doesn't exist: SELECT name, schema_version FROM {system} WHERE type = :type; Array ( [:type] => module ) in drupal_get_installed_schema_version() (line 151 of [...]/includes/install.inc).
The website encountered an unexpected error. Please try again later.
Per this report, my assumption is that this means the code is expecting the new D7 codebase because the database connection was D7 style: https://www.drupal.org/node/1308308 ]]
main_something
being missing suggests you have a table prefix configured.