I like building large apps in flat php.

I get bullied for not using OOP / Frameworks.

I don't like this.

Never feel bad for not using frameworks. Sometimes they can make things easier and quicker, not they are never truly needed, and way, way too many people rely on them. Being able to build a solution from scratch is a pro, not a con.

As far as OOP, there are positives and negatives in it. The big thing to worry about is to make sure your PHP is neatly contained and not just blended up with your HTML. Keeping it separate and clean, with clearly proposed methods, will go a long way towards helping with maintainability.

I think being able to do do is a plus, but you will save A LOT of time using a decent framework like Laravel or Yii2

I'm making a PHP backend myself with no framework or OOP. I decided to have one entry point (api.php) and pass the file and function i want to use--only the file i want is used, and only one finction is ran--that way, i can keep my php seperated neatly without worrying about the cognitive overhead of learning and conforming to a framework

It is good when you have simple backend, but when you have a complicated one, you need oop, frameworks or other than PHP languages
IMO best order to choose according to complexity of the task
PHP(simple) -> PHP(oop) -> php(frameworks) -> Python(Django/Flask) -> Ruby(Rails/Sinatra) -> Java(Play/Spring/Hibernate) -> Java EE (for suicidal development)

@OrestH well my backend is simple, so I'm good