Symfony version(s) affected

5.4.20 and above

Description

When no user agent is set FirePHPHandler::onKernelResponse throws

Deprecated: preg_match(): Passing null to parameter #2 ($subject) of type string is deprecated

in PHP 8.1 and above

https://github.com/symfony/symfony/blob/5.4/src/Symfony/Bridge/Monolog/Handler/FirePHPHandler.php#L44

How to reproduce

I have no reproducer but it is an obvious potential problem because HeaderBag::get is allowed to return null.

Possible Solution

No response

Additional Context

No response