Under PHP 7, when there's an error connecting to the database (after install), you getting the following kind of error message:

General Error
SQL ERROR [ mysqli ]

Access denied for user 'root'@'172.20.0.2' (using password: YES) [1045]

An sql error occurred while fetching this page. Please contact an administrator if this problem persists.

On PHP 8.1 and 8.2, you instead get the following:

Fatal error: Uncaught Error: mysqli object is not fully initialized in /var/www/html/phpbb/db/driver/mysqli.php:200
Stack trace:
#0 /var/www/html/phpbb/db/driver/mysqli.php(200): mysqli_query(Object(mysqli), 'SELECT config_n...')
#1 /var/www/html/phpbb/db/driver/factory.php(353): phpbb\db\driver\mysqli->sql_query('SELECT config_n...', 0)
#2 /var/www/html/phpbb/config/db.php(86): phpbb\db\driver\factory->sql_query('SELECT config_n...')
#3 /var/www/html/phpbb/config/db.php(55): phpbb\config\db->initialise(Object(phpbb\cache\driver\file))
#4 [internal function]: phpbb\config\db->__construct(Object(phpbb\db\driver\factory), Object(phpbb\cache\driver\file), 'phpbb_config')
#5 /var/www/html/vendor/symfony/dependency-injection/ContainerBuilder.php(1176): ReflectionClass->newInstanceArgs(Array)
#6 /var/www/html/vendor/symfony/dependency-injection/ContainerBuilder.php(634): Symfony\Component\DependencyInjection\ContainerBuilder->createService(Object(Symfony\Component\DependencyInjection\Definition), Array, true, 'config')
#7 /var/www/html/vendor/symfony/dependency-injection/ContainerBuilder.php(1281): Symfony\Component\DependencyInjection\ContainerBuilder->doGet('config', 1, Array, true)
#8 /var/www/html/vendor/symfony/dependency-injection/ContainerBuilder.php(1238): Symfony\Component\DependencyInjection\ContainerBuilder->doResolveServices(Object(Symfony\Component\DependencyInjection\Reference), Array, true)
#9 /var/www/html/vendor/symfony/dependency-injection/ContainerBuilder.php(1149): Symfony\Component\DependencyInjection\ContainerBuilder->doResolveServices(Array, Array, true)
#10 /var/www/html/vendor/symfony/dependency-injection/ContainerBuilder.php(634): Symfony\Component\DependencyInjection\ContainerBuilder->createService(Object(Symfony\Component\DependencyInjection\Definition), Array, true, 'cache')
#11 /var/www/html/vendor/symfony/dependency-injection/ContainerBuilder.php(588): Symfony\Component\DependencyInjection\ContainerBuilder->doGet('cache', 1)
#12 /var/www/html/includes/compatibility_globals.php(45): Symfony\Component\DependencyInjection\ContainerBuilder->get('cache')
#13 /var/www/html/common.php(154): register_compatibility_globals() #14 /var/www/html/index.php(23): include('/var/www/html/c...')
#15
Unknown macro: {main}
thrown in /var/www/html/phpbb/db/driver/mysqli.php on line 200

Those errors come from installing phpBB and then changing the database password in config.php to an incorrect one.

The PHP 8 error messages are a bit difficult to decipher and unclear as to the actual problem.