As others have said, approaching this with the mindset of "this all needs to be rewritten!" is counter productive, and will set you up on collision course with

As others have said, approaching this with the mindset of "this all needs to be rewritten!" is counter productive, and will set you up on collision course with leadership. I inherited something similar 12 years ago, also cobbled together PHP, also no separation of code and rendering - making any sort of progress was painful. As others have said there are a myriad of ways to extend code like this, encapsulating the old with a better facade. Splitting some pieces off - but it needs to be approached as a piecemeal project that takes a decent amount of time, but can be done in parallel with shipping new features.