White screen of death with undefined function _system_theme_data() in drupal 6 after update

This similar-but-different issue - https://www.drupal.org/project/drupal/issues/766100 - made me think it was an incompatibility with Drupal 7.

But this exact same error:

update.php WSOD when settings.php’s $db_url or $db_prefix are wrong https://www.drupal.org/project/drupal/issues/281997


And this general information - https://stackoverflow.com/questions/44403451/is-drupal-6-is-compatible-with-php-7 - and all recent LTS releases of Drupal 6, which is what i upgraded to, are compatible with PHP 7 - makes me think the problem was somehow with the database credentials?

https://www.mydropwizard.com/categories/drupal-6-security-update https://github.com/d6lts/drupal/releases/tag/6.52

Anyhow it seems i’d have to install a different version of Drush to use Drush, and with that alone it’s probably better to ignore the Drupal 6 site and work on the upgrade.