Installer: 'db_prefix' in database configuration form renamed to 'prefix'

The "Set up database" configuration form of the Drupal installer still used a 'db_prefix' form input name, even though the database connection info array key for the table prefix is 'prefix' since Drupal 7.

If your custom code manually submits the database configuration form of the Drupal installer, you need to update the passed form values accordingly:

Drupal 7

$parameters['forms']['install_settings_form']['mysql']['db_prefix'] = 'custom_';
// ...
install_drupal($parameters);

Drupal 8

$parameters['forms']['install_settings_form']['mysql']['prefix'] = 'custom_';
// ...
install_drupal($parameters);