What are analysis levels in PHPStan, and why would a project typically start at a lower level before gradually increasing strictness?

PHPStan organizes its checks into numbered levels, starting from a lenient level zero that only catches the most obvious, clear cut errors, up to a very strict maximum level that enforces comprehensive type correctness throughout the entire codebase, and a project with a large amount of existing code that was not originally written with static analysis in mind typically starts at a lower level to avoid being immediately overwhelmed by a huge number of reported issues, then gradually increases the level over time as the team fixes existing issues and becomes more comfortable with stricter type discipline.

# phpstan.neon
parameters:
    level: 5
    paths:
        - src