PHPStan allows you to declare the currently reported list of errors as “the baseline” and cause it not being reported in subsequent runs. It allows you to be interested in violations only in new and changed code.

It works best when you want to get rid of a few dozen to a few hundred reported errors that you don’t have time or energy to deal with right now. It’s not the best tool when you have 15,000 errors — you should probably spend more time configuring PHPStan or run it with a lower rule level.