If MySQL PDO driver is absent (that is, pdo_mysql PHP extension is not enabled), phpBB throws PHP errors like:

[critical] Uncaught Error: Undefined constant PDO::MYSQL_ATTR_FOUND_ROWS, referer: [ROOT]/adm/index.php?i=acp_board&mode=email

The problematic code is [ROOT]\phpbb\db\doctrine\connection_parameter_factory.php, function enrich_parameters() which refers to \PDO::MYSQL_ATTR_FOUND_ROWS without additional checks.

For the reference: https://www.php.net/manual/en/ref.pdo-mysql.php#pdo.constants.mysql-attr-found-rows