Container used before initialization causes error in PHP8

I have a fresh phpbb3.3.4 installation. I did nothing special and i got a HTTP 500 error and i couldn t entered my forum and not my acp
I contacted the host . I checked my server error log and i find this :

[code]FastCGI: server "/xxx stderr: #6 /www/vendor/symf... error: Uncaught Error: Call to a member function getParameter() on null in /www/includes/functions.php:2999
FastCGI: server "/xxxx stderr: #5 /www/vendor/symfony/dependency-injection/Loader/YamlFileLoader.php(127): Symfony\\Component\\DependencyInjection
Loader
YamlFileLoader->parseImports(Array, '/www/config/pro...'),
FastCGI: server "/xxx: #4 /www/vendor/symfony/dependency-injection/Loader/YamlFileLoader.php(195): Symfony\\Component\\Config
Loader
FileLoader->import('../../default/c...', NULL, false, '/www/config/pro...'),
FastCGI: server "/xxx stderr: #3 /www/vendor/symfony/config/Loader/FileLoader.php(90): Symfony\\Component\\Config
Loader
FileLoader->doImport('../../default/c...', NULL, false, '/www/config/pro...'),
FastCGI: server "/xxx" stderr: #2 /www/vendor/symfony/config/Loader/FileLoader.php(136): Symfony\\Component
Config
FileLocator->locate('../../default/c...', '/www/config/pro...', false),
FastCGI: server "/xxx" stderr: #1 /www/vendor/symfony/config/FileLocator.php(60): file_exists('/www/config/../...'),
[/code]

Answer of my host : " it is probably because of your PHP version. In your control panel, go to your domain > manage > PHP options > lower it to 7.3

Then wait about 10 minutes and test your site again. It most likely has to do with a CMS update that doesn't work well with the chosen PHP version.

CMS is the core of your website. In your case this appears to be 'symfony' based on your error outputs."

I have replaced the PHP 8 to PHP 7.3 an i replaced symfony of the original download PHPBB3.3.4 but after resett PHP 8 again, the same errors occures

How i can get back PHP 8?