(No one can make you use them, but it’s good to be aware of them)

Junior Level

PSR-1 basic coding standard (*)

PSR-2 coding style guide (deprecated)

PSR-12: Extended Coding Style (recommended) (*)

PHP - the right way

Clean Code, adapted for PHP

Middle Level

Pros and cons of getting configs from .env files and from code.

The Twelve Factors App

Object Calisthenics by William Durand

Best practices from TheCodingMachine

Senior Level

The 2018 Guide to Building Secure PHP Software