The refactoring of PHP-FPM logging has been merged and will be part of PHP 7.3.
I just merged my PR for PHP-FPM tests that greatly simplifies all tests in FPM and fixes existing failing tests.
I have started refactoring of zlog (internal FPM logger) to allow better handling of user output as it is also used for catching stdout and stderr from workers.
I have been interested in PHP-FPM for some time and currently looking to various issues in it. It is basically a PHP FastCGI application that is consisting of a master process that is managing workers that execute PHP scripts.