Symfony version(s) affected: 4.4
The problem started after updating SF4.3 to 4.4 but I'm not really sure why earlier versions would not be affected. I am running php 7.3.11.

Description
When using the ChromePHPHandler the following error may occur:

Notice: Accessing static property Symfony\Bridge\Monolog\Handler\ChromePhpHandler::$sendHeaders as non static in ChromePhpHandler.php line 62

To replicate add the following to the monolog configuration:

monolog:
    handlers:
        chromephp:
            type: chromephp
            level: debug

I think we can replace the $this->sendHeaders on lines 44 and 62 with self::$sendHeaders without any problem since the sendHeaders property is static in both version 1.25.1 and 2.x of monolog.

PR is on its way.